The ISO 20022 Migration for CHAPS and RTGS

In June 2023, the Bank of England migrated CHAPS, the UK's real-time gross settlement rail, from legacy SWIFT MT messages to the ISO 20022 standard. It was one of the most significant plumbing changes in UK payments in a generation, and it sits inside a broader global shift away from MT messaging. This post explains what actually changed and why it matters.

Why ISO 20022

The MT format that underpinned high-value payments for decades is compact but constrained: fields are short, loosely structured, and often stuffed with free text that machines cannot reliably parse. ISO 20022 replaces this with structured, XML-based messages that carry richer, clearly defined data. For a credit transfer, the equivalent message is pacs.008 (FIToFICustomerCreditTransfer); status updates use pacs.002; returns use pacs.004.

The richer data is the whole point. ISO 20022 supports fully structured names and addresses, dedicated purpose codes, Legal Entity Identifiers (LEIs), and structured remittance information. That means better fraud detection, cleaner sanctions screening, straight-through reconciliation, and fewer payments held up because a name or reference was buried in free text.

The Bank of England RTGS renewal

The CHAPS migration was one strand of the Bank of England's multi-year RTGS Renewal Programme, which is rebuilding the core settlement infrastructure that holds sterling reserves and settles CHAPS. Alongside the messaging change, the renewal aims to increase resilience, extend access to a wider range of firms, improve interoperability with other systems, and support future features such as synchronised settlement.

The migration path: enhanced data in stages

Rather than switching everything at once, the migration proceeded in phases. The first step was moving to ISO 20022 messages that were like-for-like with the old MT content, so participants could adopt the new format without immediately reworking every data field. Subsequent phases mandated enhanced data: from later milestones, CHAPS payments were required to include structured elements such as purpose codes for certain payment types, LEIs for financial-institution-to-financial-institution transfers, and structured remittance and address data, with fully structured addresses becoming mandatory over a defined timeline.

The value of ISO 20022 is not the XML. It is the discipline of putting the right data in the right, machine-readable field, every time.

SWIFT coexistence and the end of MT

Globally, cross-border payments over SWIFT entered a coexistence period in March 2023, during which both MT and ISO 20022 (MX) messages could be used for cross-border and correspondent banking flows. That coexistence window was set to close in November 2025, after which the legacy MT messages for these payment categories are retired in favour of ISO 20022. The CHAPS migration was timed to align the UK's domestic high-value rail with this global direction, so that cross-border and domestic messaging speak the same structured language.

What it means for participants

For banks and their vendors, the migration was far more than a format swap. It required:

• Data quality work to source and structure names, addresses, LEIs and purpose codes that were previously optional or embedded in free text.
• Translation and coexistence handling so that systems still on MT could interoperate during transition without truncating the richer MX data.
• Screening and monitoring upgrades to take advantage of, and correctly parse, the new structured fields.
• Reconciliation improvements as structured remittance data enabled higher straight-through processing rates.

The recurring risk across the industry has been truncation: when a rich ISO 20022 message passes through a system that still thinks in MT, data can be dropped, undermining the very benefits the migration promised. Preserving data end to end is the ongoing challenge.

The payoff: screening, reconciliation and analytics

The reason regulators and central banks pushed for enhanced data is that it makes payments both safer and more useful. Structured names and addresses let sanctions-screening engines compare like with like instead of parsing free text, cutting false positives. Purpose codes and LEIs let supervisors and banks understand what payments are actually for, improving fraud detection and economic analysis. And structured remittance data lets corporates reconcile incoming payments to invoices automatically, raising straight-through processing rates. In other words, the migration was never about XML for its own sake; it was about turning opaque payment messages into structured, queryable data that downstream systems can trust.

Key takeaways

• CHAPS migrated to ISO 20022 in June 2023 as part of the Bank of England's RTGS Renewal Programme.
• Credit transfers use pacs.008, status reports use pacs.002, and returns use pacs.004.
• Enhanced data (purpose codes, LEIs, structured addresses and remittance) is being phased in as mandatory.
• SWIFT cross-border MT and MX coexistence ran from March 2023 and was set to end in November 2025.
• The main operational risk is data truncation when rich MX messages traverse legacy MT systems.