Most firms preparing for the move to T+1 are beginning with the wrong question. They want to know when testing needs to begin, when the real question is whether they understand their settlement service well enough to test it at all. For many firms, the answer is still no. The irony is that the industry already has a methodology designed to create exactly that understanding. It has simply not been applied to T+1.
The answer lies in the Operational Resilience framework. Firms have already built the tools required. They just have not yet recognised that those tools are precisely what T+1 requires.
The Methodology Already Exists. It Created Your IBS Maps.
Between 2021 and 2025, firms across the UK financial sector undertook a detailed programme under the FCA and PRA’s Operational Resilience regime. They were required to identify their Important Business Services, meaning the services where disruption would cause intolerable harm to clients or market integrity. They then had to map every dependency involved in delivering those services. That included people, processes, technology, data, and third parties.
The work did not stop there. Firms also had to scenario test the services against disruption, define impact tolerances, and demonstrate an ability to operate within those tolerances under stress. For many organisations, this was the most comprehensive review of their operating model they had ever conducted.
At a structural level, this is identical to what firms now need for T+1 readiness. The industry has simply not yet made the connection.
The EU and UK T+1 Testing Taskforce has outlined eight trade flow scenarios that firms need to prepare for. These include on exchange cleared activity with and without Power of Attorney, uncleared on exchange activity, bilateral OTC, securities lending, repos, FX, and corporate events. In Operational Resilience terms, each of these functions as a service or a sub‑service. Each has its own end to end flow, dependencies, handoffs, and a defined timeline within which completion is required.
Under T+2, there is enough slack in the process to absorb manual interventions, late matching, and delays in instruction submission. Overnight batches run, and exceptions are resolved the following morning. Under T+1, this flexibility disappears. The same operational service, with the same technology, the same data inputs, the same teams, and the same third parties now needs to complete in half the time. The stress being applied is not a cyber attack or an outage. It is time.
Time compression is a predictable stress scenario, and IBS mapping is designed specifically to highlight the points where a service will fail when put under that type of pressure.
Using the IBS lens for T+1 means asking where manual actions add latency, which third‑party cut offs sit inside the T+1 deadline, where the process collapses when a single input arrives late, and what the real impact tolerance of the settlement service is. These are operational resilience questions being applied to a new regulatory event.
The UK Data Shows Where the Service Is Already Breaking Down
The Taskforce is working with UK equity settlement data from September 2025. It gives a clear view of where firms already have structural weaknesses. Instruction submission reached 91.5 percent by the end of the trade date and 99 percent by T plus 1. Matching rates at the CSD were 86.6 percent by the end of T and 98.4 percent by T plus 1. Settlement efficiency at the Intended Settlement Date stood at 94.4 percent by value.
Interpreted through an operational resilience lens, these figures show that firms currently tolerate an 8.5 percent instruction submission failure rate on trade date and a 13.4 percent matching failure rate. Under T plus 1, there is no following day available to resolve these. They convert immediately into same day settlement fails.
Impact tolerances for settlement have rarely been formally defined, but they will be tested live during the transition whether firms are ready or not.
The experience in the United States illustrates the challenge. In January 2024, DTCC reported that only 69 percent of trades were affirmed by 9pm on trade date, significantly short of the 90 percent target for the May 2024 go live. The industry eventually met the requirement, but it required intense effort in the final months. Europe is facing the same challenge with additional regulatory pressure from ESMA, which requires allocation and confirmation to be completed by 23:00 CET on trade date from 7 December 2026. This deadline comes before the first market wide testing window in February 2027.
Firms that lack visibility of their current instruction and matching performance, by product, by market, and by counterparty, do not yet know where their settlement service breaks under compression. That analysis is the same gap assessment required by operational resilience. It needs to be carried out immediately.
Scenario Testing Is Not New. Only the Stress Variable Has Changed.
The T+1 Testing Taskforce has defined five coordinated market wide testing windows for 2027. These will be important, but the Taskforce has been explicit that readiness cannot wait for them. Much of the required preparation must occur before firms enter an FMI environment.
Operational resilience already provides a full scenario testing framework. IBS testing does not require an external environment. It requires defining a plausible scenario, tracing it through the service map, and identifying the point at which the service breaches impact tolerance.
For T+1, the scenarios are obvious. Firms should understand the impact of a custodian submitting instructions ninety minutes late, the effect of latency on a confirmation platform at 22:00 CET, and how the repo book behaves when the Settlement Optimisation Gating Event, which is due to be introduced with the ISO Standards Release in November 2026, interacts with unilateral decisions by counterparties.
These are predictable operational events that firms must test internally long before the first FMI window opens. The firms that perform well in Windows One through Five will be the firms that already understand their own weaknesses. Firms that arrive at Window One to discover them will struggle to remediate in time.
Two Workstreams Must Now Be Brought Together
A governance issue sits at the heart of many firms’ T+1 challenges. In most organisations, the teams responsible for Operational Resilience and IBS mapping are not the same teams responsible for T+1 transition and post‑trade operations. Settlement teams, project offices, and T+1 working groups are progressing their own streams, while Operational Resilience teams are maintaining frameworks and preparing for supervisory reviews.
Few firms have brought these groups together to ask whether their existing IBS documentation already contains the service map required for T+1 readiness.
In many cases, it does. Settlement and post‑trade processes are frequently mapped as Important Business Services or captured as critical components within wider services. Dependency maps, third‑party registers, and scenario testing methodologies already exist. What is missing is the application of this infrastructure to the T+1 timeline.
Firms that make this connection now will accelerate their preparation. Firms that do not will either duplicate the work they have already completed or risk designing T+1 testing on foundations that their own operational resilience documentation would show are inadequate.
How DC Is Helping Clients Prepare
DC has significant expertise in post‑trade operations, operational resilience, and regulatory change programmes. We work with brokers, asset managers, custodians, and intermediaries across the UK and EU. Our approach to T+1 begins with the service map, not the industry test window.
We bring operational resilience and T+1 teams together and apply IBS methodology directly to settlement flows. This includes aligning IBS mapping to the eight Taskforce scenarios, benchmarking firm level metrics against emerging industry data, identifying break points under time compression, designing internal scenario tests ahead of market wide windows, and developing firm specific plans for FMI engagement across OTC, securities lending, repos, and corporate events where generic guidance does not go far enough.
The timeline is fixed. ESMA’s allocation and confirmation deadline arrives in December 2026. Industry wide testing begins in February 2027. T+1 goes live in October 2027. For any firm that has not yet mobilised, the preparation window is already short.
The methodology to address these challenges already exists. Most firms have already invested in building it. The question now is whether they will make use of it.