# UBL vs CII: which e-invoice format should UK businesses use?
> UBL and CII are the two XML syntaxes for EN 16931 e-invoices. How they differ, which one Peppol requires, and what it means for UK businesses before 2029.
- Canonical URL: https://einvoices.uk/news/ubl-vs-cii-uk
- Published: 20 October 2026 (2026-10-20)
- Last updated: 20 October 2026 (2026-10-20) (not updated since publication)
- Author: Floris Lindenaar, Editor, e-invoicing expert (https://einvoices.uk/authors/floris-lindenaar)
- Publisher: einvoices.uk (https://einvoices.uk)
- Type: Explainer
- Topics: Technical, Peppol, Software, 2029 mandate
- Language: en-GB (UK English); covers the United Kingdom
## Sources
- [Navigating the eInvoicing standard documentation – European Commission](https://ec.europa.eu/digital-building-blocks/sites/display/DIGITAL/Navigating%2Bthe%2BeInvoicing%2Bstandard%2Bdocumentation)
- [Peppol BIS Billing 3.0 (May 2026 release, version 3.0.21)](https://docs.peppol.eu/poacc/billing/3.0/bis/)
- [ConnectingEurope/eInvoicing-EN16931 validation artefacts](https://github.com/ConnectingEurope/eInvoicing-EN16931)
- [Factur-X – FNFE-MPE](https://fnfe-mpe.org/factur-x/)
- [Promoting electronic invoicing across UK businesses and the public sector: consultation response](https://www.gov.uk/government/consultations/promoting-electronic-invoicing-across-uk-businesses-and-the-public-sector/outcome/promoting-electronic-invoicing-across-uk-businesses-and-the-public-sector-consultation-response)
- [Tax update 2026: simplification, modernisation and fairness summary – GOV.UK](https://www.gov.uk/government/publications/summary-of-tax-update-2026-simplification-modernisation-and-fairness/tax-update-2026-simplification-modernisation-and-fairness-summary)
---
UBL and CII are the two XML formats (syntaxes) that can carry an invoice built to the European standard EN 16931. For UK businesses the practical answer is **UBL**: Peppol, which the government has confirmed as the UK's e-invoicing network, uses UBL 2.1 as its mandatory format and treats CII as optional. The UK data standard for the 2029 mandate has not been published, so that is the best indication today, not a confirmed rule.
## Same data, two syntaxes
[EN 16931](https://einvoices.uk/news/en-16931-explained) splits an e-invoice into two layers:
1. **The semantic model** (EN 16931-1): the list of business terms an invoice can contain, such as BT-1 Invoice number and BT-110 Invoice total VAT amount.
2. **The syntax**: how those terms are written as a file.
The European Commission's guide lists the bindings. CEN/TS 16931-3-2 covers "ISO/IEC 19845 (UBL 2.1) invoice and credit note". CEN/TS 16931-3-3 covers "UN/CEFACT XML Industry Invoice D16B". The Commission says public bodies covered by Directive 2014/55/EU must be able to receive and process both.
So a UBL invoice and a CII invoice carrying the same data are equivalent at the semantic level. The European Commission's validation artefacts on GitHub ship separate rule sets for each, with the same business rule IDs, so BR-CO-15 means the same thing in both.
## How they differ in practice
| | UBL 2.1 | UN/CEFACT CII D16B |
|---|---|---|
| Maintained by | OASIS (also ISO/IEC 19845) | UN/CEFACT |
| Document types | Separate `Invoice` and `CreditNote` | One `CrossIndustryInvoice`; type code sets invoice or credit note |
| Root namespace | `urn:oasis:names:specification:ubl:schema:xsd:Invoice-2` | `urn:un:unece:uncefact:data:standard:CrossIndustryInvoice:100` |
| Structure | Fairly flat; parties and totals near the top | Nested in header blocks: agreement, delivery, settlement |
| Peppol BIS Billing 3.0 | Mandatory | Optional |
| Where you see it | The Peppol network, including invoices to the NHS | The XML inside Factur-X / ZUGFeRD hybrid PDFs |
The same invoice number looks like this in each.
UBL:
```xml
INV-2026-0142
2026-10-20
380
```
CII:
```xml
INV-2026-0142
380
20261020
```
These are fragments to show structure, not complete valid invoices. Note the date difference: UBL uses ISO dates (`2026-10-20`); CII uses a format code with `20261020`.
## What Peppol says about CII
Peppol BIS Billing 3.0 is specified in UBL. The schemas are "UBL Invoice 2.1" and "UBL CreditNote 2.1". Appendix B adds CII as an option:
> "This Peppol BIS supports the optional use of the UN/CEFACT XML Cross Industry Invoice, D16B (CII), meaning that receivers of invoices can register in the SMP to receive CII invoices alongside the mandatory UBL version."
Two consequences:
- **Every Peppol receiver must accept UBL.** If you send UBL, any Peppol participant can take it.
- **CII only works if the receiver has registered for it.** Sending CII to someone who has not is a delivery problem, not just a validation one.
Peppol also says a CII invoice must meet the same Peppol rules as a UBL one, with the same specification identifier.
## Where CII still matters for UK businesses
- **Factur-X and ZUGFeRD.** FNFE-MPE describes Factur-X as a Franco-German hybrid format: a PDF for people plus XML for machines. It says "Factur-X et ZUGFeRD sont identiques" ("Factur-X and ZUGFeRD are identical"). The XML is CII. UK businesses with French or German customers may already receive these.
- **ERP output.** Some ERP e-invoicing modules produce CII natively for EU countries. Converting to UBL is usually done by the Access Point or the software.
- **Archive and import.** If you receive both, your AP system needs to read both.
## What the UK has and has not decided
**Confirmed:** "The government has announced that the electronic procurement system Peppol will be the core interoperability network for e-invoicing in the UK" (GOV.UK, Tax update 2026, 23 June 2026). The mandate starts on April 2029.
**Not yet confirmed:** the UK data standard and syntax. In its consultation response, the government said it "will work closely with stakeholders to review the risks and benefits of adopting an existing standard or designing a UK-specific standard". Peppol's mandatory syntax is UBL, so UBL is the safe bet, but whether CII will be accepted for UK mandate purposes has not been announced. Expect more detail in the roadmap due at Budget 2026 on 28 October 2026.
## So which should you pick?
- **Most businesses:** you will not pick. Your software and [Access Point](https://einvoices.uk/glossary#access-point) handle it. Ask your provider whether they send and receive Peppol BIS Billing 3.0 UBL today.
- **Developers building an integration:** build UBL first. It is the one every Peppol receiver must accept, and the format we use in our [UK Peppol invoice XML example](https://einvoices.uk/news/e-invoice-xml-example-uk).
- **Businesses trading with France or Germany:** make sure you can also receive CII-based Factur-X / ZUGFeRD.
## What to do now
- Ask your software provider which syntaxes it can generate and receive.
- If you have in-house integrations, check that they map to EN 16931 business terms, not just one vendor's format. That makes switching syntax far easier.
- Read our [Peppol BIS Billing 3.0 UK guide](https://einvoices.uk/news/peppol-bis-billing-3-uk-guide) next.
## Frequently asked questions
### What is the difference between UBL and CII?
Both are XML syntaxes that can carry an EN 16931 invoice. UBL 2.1 is an OASIS standard (ISO/IEC 19845) with separate Invoice and CreditNote documents. CII is UN/CEFACT's Cross Industry Invoice (D16B), with one document type for both.
### Does Peppol use UBL or CII?
Peppol BIS Billing 3.0 is built on UBL 2.1. UBL support is mandatory. CII is optional: receivers can register to accept CII invoices alongside UBL.
### Which format will the UK mandate require?
Not confirmed. Peppol is confirmed as the UK's network, which points strongly to UBL. The UK data standard has not been published.
### Do I need to choose between UBL and CII myself?
Usually not. Your accounting software or Access Point generates the XML. You may need to know the format if you build integrations or receive invoices directly.
---
How to cite: "UBL vs CII: which e-invoice format should UK businesses use?", einvoices.uk, 20 October 2026, https://einvoices.uk/news/ubl-vs-cii-uk
einvoices.uk is an independent publication and is not affiliated with HMRC or the UK Government. Nothing here is tax advice; check the sources and official GOV.UK guidance before acting.