The four-corner model of e-invoicing explained

The four-corner model is the way most modern e-invoicing networks, including Peppol, are organised. The supplier and the buyer each pick their own service provider, and the providers exchange invoices with each other over a common network. The UK's confirmed approach, a decentralised model with Peppol as the core interoperability network, is a four-corner model.

The four corners

Corner	Who	Role in a UK example
C1	Supplier	A Sheffield engineering firm raises an invoice in its accounting software
C2	Supplier's service provider (Access Point)	Validates the invoice, looks up the buyer, sends it over Peppol
C3	Buyer's service provider (Access Point)	Receives the invoice, checks it, passes it on
C4	Buyer	A Glasgow retailer's ERP imports the invoice for approval

The supplier never needs to know which provider the buyer uses. C2 finds out automatically by looking up the buyer's Peppol ID (see SMP and SML explained).

Step by step

1. C1 creates a structured invoice, for example Peppol BIS Billing 3.0 in UBL XML. It is not a PDF.
2. C2 validates it against the rules and rejects it if it fails.
3. C2 looks up the buyer's Peppol ID to find C3's address and certificate.
4. C2 sends the invoice securely to C3 using the AS4 protocol.
5. C3 checks it and delivers it to C4. If something fails, C3 sends a Message Level Status back to C2 (see Invoice Response vs MLR/MLS).
6. C4 processes the invoice, ideally matching it to the order automatically.

On Peppol, C2 and C3 have both signed the same Peppol agreements and passed conformance testing, which is what lets strangers trust each other.

Why four corners and not one platform?

The Department of Health's NHS eProcurement Strategy (April 2014), which brought Peppol into the NHS, explained the model simply:

• "each NHS provider selects its own access point for messaging all suppliers"
• "each supplier selects its own access point for messaging all NHS Providers"
• "There are no fees payable between access points, with each party paying only their own PEPPOL access point provider."

Compared with alternatives:

Model	How it works	Main drawback
Two-corner (direct)	Supplier connects straight to buyer, by EDI link, portal upload or email	One connection per trading partner
Three-corner	One platform serves both sides	Everyone must join the same platform; risk of lock-in
Four-corner	Each side chooses a provider, and providers interconnect	Needs common rules and governance (Peppol supplies these)
Five-corner	Four-corner plus reporting to the tax authority	More data flows and obligations
Centralised / clearance	Invoices pass through a government platform	Dependence on one platform; heavier state role

OpenPeppol contrasts its approach with "closed or 'three-corner' networks". The four-corner model's selling point is that, like email, one connection reaches everyone.

Four corners vs five corners

In a five-corner model, invoice data is also sent to the tax authority, the fifth corner, usually by the service providers. The Peppol Network Policy treats a five-corner model as "a combination of multiple instances of a 4-Corner Model". Technically, the fifth corner is another Peppol exchange.

In the government's 2025 consultation response, several respondents recommended "considering the Peppol network on either a four-corner or five-corner model". The government's decision for launch: no real-time reporting alongside the mandate. So the UK starts as a four-corner model. Whether a reporting element (a fifth corner) is added later has not been confirmed. See Five-corner model and the UK and Will HMRC require real-time reporting?

What it means for different businesses

• Small businesses: you will usually get corner 2 or 3 through your accounting software. You do not need to "join" Peppol separately. See the small business guide.
• Larger businesses: you may connect your ERP to a provider by API or file transfer. Pick the provider on certification, validation, portability and price (see How to choose a Peppol Access Point).
• Public sector suppliers: many NHS suppliers already work this way.
• Businesses that receive invoices on supplier portals: the four-corner model can replace dozens of portal logins with one connection, if your customers support it.

What is confirmed and what is not

CONFIRMED (official):

• Peppol "will be the core interoperability network for e-invoicing in the UK" (GOV.UK Tax Update summary, 23 June 2026).
• Decentralised model, with mandatory e-invoicing for B2B and B2G VAT invoices from April 2029.
• No real-time reporting at launch.

NOT YET CONFIRMED (as of 20 October 2026):

• how service providers will be accredited for the UK mandate
• what role, if any, non-Peppol channels such as existing EDI or legacy systems will have. The government says it "will continue to engage with stakeholders regarding the role of legacy systems which cannot interoperate in the future system".
• whether and when any reporting to HMRC (a fifth corner) will follow

The implementation roadmap is due at Budget 2026 on 28 October.

What to do now

1. Identify your corner 2 and 3. Ask your software vendor which certified Peppol provider it uses.
2. Check that your provider can both send and receive.
3. Find your main customers' and suppliers' Peppol IDs in the Peppol Directory.
4. Read What is Peppol? and the glossary entry for the four-corner model.