“Why Has My Invoice Been Rejected?” PO Numbers for Tradespeople Explained

What is a PO number?

A Purchase Order number, usually shortened to PO number, is a reference generated by the customer's purchasing or procurement process.

It might look something like:

PO562894

The important point is that the PO normally comes from your customer, not from your own invoice numbering system.

It allows the customer's finance team to connect your invoice with an authorised purchase.

Conceptually:

Customer authorises purchase

↓

Purchase Order created

↓

PO number issued

↓

Work / goods supplied

↓

Supplier invoice references PO

↓

Accounts matches invoice to authorised purchase

↓

Payment processing

The Small Business Commissioner's invoicing guidance says a PO number is a reference supplied by the customer and ideally issued before work or services begin. Where one has been issued, it recommends including it on the invoice because it helps the customer match the invoice to the work and can speed up payment.

Not every customer uses purchase orders

This matters.

You should not assume every domestic customer, landlord or small business needs a PO.

Many don't.

The Small Business Commissioner explicitly notes that not every business uses PO numbers.

The problem becomes more common when you work with organisations that have formal procurement or accounts-payable processes.

For example:

• facilities-management companies;
• housing organisations;
• councils;
• larger construction contractors;
• corporate maintenance departments;
• public bodies;
• multi-site businesses.

So the useful question when a commercial customer books work is:

“Do you require a purchase order number on the invoice?”

That five-second question can save weeks of payment delay later.

What does “No PO, No Pay” mean?

Some organisations operate a formal:

No PO, No Pay

policy.

In simple terms:

No valid purchase order number on the invoice → invoice is not processed for payment.

This isn't theoretical.

The Electoral Commission states that invoices without a valid PO number are returned unpaid and tells suppliers to contact the person who requested the goods, services or works to obtain the PO and then resend the invoice.

Hackney Council similarly says that invoices containing no PO or an invalid PO will be rejected under its No PO, No Pay rule.

Westminster City Council says invoices without a valid and correct PO number will be rejected and returned.

And the Insolvency Service says invoices without a valid PO may be returned and, in those cases, its payment lead time does not begin until a valid invoice has been received.

That last point is particularly painful.

You may think:

“They're already seven days into my 30-day terms.”

Their accounts system may effectively be saying:

“We haven't accepted a valid invoice yet.”

That exact consequence depends on the customer's terms and procurement process, so don't assume it applies universally.

This problem is being discussed by UK small businesses now

In June 2026, a UK small-business owner described a customer insisting:

“NO PO – NO PAY”

for future invoices.

The business issued many relatively small invoices and was trying to understand whether a customer's internal procurement requirement could prevent payment where the service had already been provided. The discussion attracted more than 160 votes and substantial debate.

The important practical takeaway isn't to settle every possible legal dispute through Reddit.

It's this:

If a commercial customer has told you they require a PO, don't knowingly send an invoice without it.

Deal with the requirement before it becomes a payment dispute.

Our example: the £1,500 boiler invoice

Let's use the real Love Invoicing workflow you created.

The tradesperson completes:

Boiler Fitting

The invoice contains:

Subtotal: £1,250
VAT at 20%: £250
Total: £1,500
Payment Terms: Net 7

The invoice is sent to ACME LTD.

But one important field is empty:

PO Number

“But Love Invoicing says PO Number optional”

Correct.

And it should.

A PO number isn't universally required.

The field being optional in the software does not mean it is optional under every customer's purchasing process.

Think of it this way:

Software requirement:
Can an invoice technically be created without a PO?

Yes.

Customer requirement:
Will this particular customer's accounts team process it without a PO?

Maybe not.

That is why the business needs to understand the customer's invoicing requirements.

The invoice is sent

The example invoice becomes:

INV-2026-0026

ACME LTD

£1,500

Sent

Balance due: £1,500

Everything appears normal.

Until accounts replies.

Then the email arrives: “PO number missing”

Imagine ACME LTD replies:

Invoice rejected — please include PO562894 and resubmit.

This is where many small businesses lose unnecessary time.

Someone now has to:

1. contact the customer;
2. obtain or confirm the PO;
3. find the invoice;
4. correct it;
5. resend it;
6. ensure accounts uses the corrected version.

The Small Business Commissioner recommends avoiding this situation by requesting required PO numbers early, because delays obtaining them can delay invoice submission and payment.

Its current effective-invoicing guide even gives a real case where a business had an invoice worth more than £12,000 outstanding because the PO number had not been included. After the invoice was reissued with the correct PO, payment was received later that month.

That is almost exactly the scenario we're demonstrating.

So what do you do if the invoice has already been sent?

Don't pretend the original invoice never happened.

Correct the record deliberately.

In Love Invoicing, the sent invoice can be edited.

From the invoice menu, select:

Edit

The app then warns:

This invoice has already been sent. We'll keep the original saved and create an updated version for your records.

That matters because there is now a distinction between:

what was originally sent

and:

what was subsequently corrected.

Add the PO number

The customer supplies:

PO562894

The tradesperson adds it to the revised invoice.

The updated reference becomes:

INV-2026-0026-R1

In this example, R1 represents the first revision.

If another revision were subsequently needed, the revision sequence can continue.

Resend the corrected invoice

After saving and sending, the revised invoice now shows:

INV-2026-0026-R1

and:

PO Number: PO562894

The customer receives the corrected version with the purchasing reference their accounts process requires.

Why keep the original invoice?

Because once a document has already been sent externally, simply overwriting history makes it harder to answer basic questions later.

For example:

Which version did accounts originally receive?

When was the PO added?

Was the £1,500 amount changed?

Which document is current?

Your web screenshot demonstrates the Love Invoicing approach clearly:

Version History

INV-2026-0026 — Superseded — Original

INV-2026-0026-R1 — Sent — Current

That gives the business a record of both versions.

Important wording for the article:

This provides a useful invoice version history within Love Invoicing. Whether a particular revision, cancellation, credit-note or accounting treatment is appropriate can depend on the nature of the change and the accounting/tax circumstances.

We should not make the broader claim that every invoice correction in UK accounting should always be made using R1/R2 revisions.

That's too broad.

Here, we're demonstrating how Love Invoicing preserves its record when a sent invoice is revised.

Should you simply edit every invoice after a customer complains?

No.

First identify what is actually wrong.

There is a substantial difference between:

Missing administrative information

Example:

PO number missing

and:

The commercial invoice itself is wrong

For example:

• wrong quantity;
• incorrect work;
• incorrect VAT;
• wrong customer;
• duplicate charge;
• incorrect total;
• payment already allocated incorrectly.

Those situations may need a different accounting treatment.

For VAT invoices specifically, HMRC has rules around correcting errors and when credit notes or other corrections may be required. Don't assume that an R1 revision inside software replaces any tax or accounting action that may otherwise be required.

For this article, the example is deliberately narrow:

Same £1,500 invoice → missing customer PO added → corrected version resent.

A PO should ideally be obtained before the work begins

Fixing the invoice works.

Preventing the problem is better.

The Small Business Commissioner says that if a PO number is required, businesses should request it early to avoid invoice-submission delays.

The Electoral Commission goes further under its own procurement policy and tells suppliers not to accept an order without an order number.

Hackney similarly states that its PO needs to be raised and approved before goods or services are delivered.

These are organisation-specific procurement requirements rather than a universal rule for every customer in Britain.

But they illustrate the habit trade businesses should develop:

Before accepting commercial work, establish the customer's purchasing process.

Ask these five questions before a commercial job

When a facilities manager, contractor or commercial customer rings, ask:

1. Do you require a PO?

If yes, get it.

2. Who is the legal entity I'm invoicing?

The person calling from a site may work for a different company from the entity responsible for payment.

3. Where does the invoice need to go?

Individual contact?

Accounts payable?

Procurement portal?

Dedicated invoice email?

4. What information must appear on it?

PO?

Site reference?

Job number?

Contract number?

Cost centre?

5. What are the agreed payment terms?

7 days?

30 days?

End-of-month plus 30?

Don't wait until the invoice is overdue to discover how their accounts department works.

The PO number isn't the only thing that can cause rejection

A commercial invoice can also run into problems if:

The company name is wrong

The amount differs from the PO

The line description doesn't correspond with the purchase

VAT is incorrect

The invoice goes to the wrong department

The invoice doesn't meet the required submission format

Hackney explains that mismatches between its PO, invoice and receipt information can delay payment.

The Insolvency Service requires invoices under its procurement process to map accurately to PO line descriptions, quantities and unit prices.

So:

PO number present

doesn't automatically equal:

invoice valid

It's one part of getting the commercial invoice right.

Why this matters particularly to tradespeople

A tradesperson can easily end up trapped between two departments.

Operations says:

“Please get someone here immediately.”

Accounts later says:

“Where is the PO?”

The electrician or plumber did exactly what the person on site asked.

But the customer's internal purchase authorisation may not have caught up with the operational request.

That becomes especially common with:

• emergency repairs;
• multi-site maintenance;
• commercial plumbing;
• electrical maintenance;
• property-management work;
• facilities management;
• subcontracting;
• reactive maintenance.

That is why commercial field-service businesses need to understand more than simply:

Do the job → send invoice.

They also need:

Authorisation → job → correct invoice → accounts approval → payment.

Don't confuse PO number with invoice number

They are different references.

In our example:

Your invoice reference

INV-2026-0026-R1

This identifies the invoice within your business records.

Customer's PO number

PO562894

This identifies the authorised purchase within the customer's process.

One does not replace the other.

The customer may quote:

“Can you resend invoice INV-2026-0026-R1 against PO562894?”

Both references matter.

What Love Invoicing contributes

Love Invoicing doesn't determine whether your customer's purchasing policy requires a PO.

The business still needs to ask.

What it can do is provide a place to capture the information and keep the corrected record organised.

In this example:

Create Invoice

↓

Add PO Number where required

↓

Send Invoice

↓

Customer identifies missing PO

↓

Edit Sent Invoice

↓

Original retained

↓

R1 created

↓

PO added

↓

Revised invoice resent

↓

Version History retained

That is substantially cleaner than manually editing PDFs and ending up with files called:

invoice-final.pdf

invoice-final-new.pdf

invoice-final-corrected.pdf

invoice-final-corrected-2.pdf

with nobody knowing which one the customer should pay.

Frequently Asked Questions

What is a PO number on an invoice?

A purchase order or PO number is a customer-issued reference that links an invoice to an authorised purchase. Not every customer uses purchase orders, but where one is issued it should normally be included on the relevant invoice.

What does No PO, No Pay mean?

It describes a procurement policy under which an organisation will not process an invoice unless it references a valid purchase order. Several UK public organisations explicitly operate this type of policy.

Can a customer reject an invoice because the PO number is missing?

A customer's procurement process may reject or return an invoice that does not meet its agreed requirements. Whether a customer can ultimately avoid a legal obligation to pay for work already supplied is a different question and depends on the contract and circumstances. A No PO, No Pay policy should therefore not be interpreted as a universal legal right to receive completed work without paying.

Does every tradesperson invoice need a PO number?

No. Many customers do not use purchase orders. The Small Business Commissioner says not every business issues PO numbers.

Who provides the PO number?

The customer or their procurement/purchasing process normally provides it.

When should I ask for a PO number?

Where the customer requires one, request it as early as possible, ideally before the work or service begins. The Small Business Commissioner specifically recommends requesting required PO numbers early to avoid invoice and payment delays.

What if I forgot the PO number and already sent the invoice?

Obtain the correct PO from the customer and follow the appropriate correction/reissue process for your invoicing and accounting system. In the Love Invoicing example shown here, editing the sent invoice retains the original and creates a revised R1 version containing the PO.

What does R1 mean on the Love Invoicing invoice?

In the example, R1 identifies the first revised version of the sent invoice. The original invoice remains in Version History and the revised invoice is shown as the current version.

What happens to the original invoice after it is revised in Love Invoicing?

The web example shown in this article retains INV-2026-0026 as the original, marked Superseded, while INV-2026-0026-R1 is identified as the current version.

Can I add a PO number without changing the invoice amount?

In the workflow demonstrated here, yes. The original and revised invoices both remain £1,500; the correction is the addition of the customer PO number.

Does adding the PO automatically mean the invoice will be paid?

No. The customer may have other invoice requirements, such as correct company details, descriptions, prices, VAT treatment, submission channel or approval of the work.

Does a PO number replace my invoice number?

No. Your invoice number and the customer's PO number serve different purposes.

Can missing PO numbers delay payment terms?

They can under some procurement systems. For example, the Insolvency Service says that where an invoice without a valid PO is returned, its payment lead time does not begin until a valid invoice is received.

That should not be assumed to be the policy of every UK customer.

What should tradespeople check before invoicing a commercial customer?

At minimum, confirm the correct customer entity, invoice recipient, PO requirement, work description, pricing, VAT where applicable, payment terms and any customer-specific invoice references or submission procedures.

Can Love Invoicing store a customer PO number?

Yes. The invoice form shown here includes a PO Number field, and the entered number appears on the resulting invoice record.

Can Love Invoicing revise an already-sent invoice?

The workflow shown in your screenshots allows a sent invoice to be edited while retaining the original and generating an updated revision for the records.