UK CBAM 2027 Deadlines: Transitional Rules

Your first UK CBAM deadline is probably not the one already sitting in your compliance calendar. The opening periods of the regime do not run on the ordinary timetable.

SI 2026/830 sets a transitional calendar for registration, returns and payment. The instrument is short, and its limits matter as much as its dates. One date applies only to people whose registration trigger occurs during a specified year. The return dates cover only the first annual period and the next 2 quarters.

31 January 2028 is not a general registration deadline

If your duty to register is triggered during 2027, you use 31 January 2028 instead of the ordinary 30-day registration deadlineregistration deadlineThe last date by which a person liable to register must complete UK CBAM registration..

That substitution is narrow. 31 January 2028 is not a universal UK CBAM registration deadline. It replaces the 30-day rule only for people whose trigger falls inside 2027.

The distinction has to be visible in your internal checklist. A generic line reading "registration due by January" will misstate the rule for anyone whose trigger occurs outside that transitional group.

The instrument also does not establish when an online registration service will open. The legal duty deadline and the operational launch date are separate questions.

The first accounting period runs for a full year

The first accounting periodaccounting periodThe period for which a UK CBAM return and payment are calculated. covers 1 January to 31 December 2027. The return and the payment for that period are due by 31 May 2028.

The first period therefore has an annual shape. Do not model it as 4 ordinary quarterly returns simply because quarterly periods appear later in the regime.

31 May 2028 is a filing date. It is not the date on which data collection starts. You will prepare the return after the year closes, but you need transaction, origin, weight, emissions and relief evidence gathered across the whole of 2027 to do it.

Align the 2027 import ledger, the CBAM data trail and the period-end review well before 31 December 2027.

Only 2 quarterly periods are modified

For the period ending 31 March 2028, the return and payment deadlinereturn and payment deadlineThe deadline by which both the UK CBAM return and the corresponding tax payment are due. is 31 July 2028.

For the period ending 30 June 2028, the return and payment deadline is 29 September 2028.

Those are the only 2 quarterly periods the instrument expressly modifies. The temporary charge and accounting modifications concern charges arising from 1 January 2027 through 30 June 2028. The return modifications cover periods ending by 30 June 2028.

Do not project a later quarterly calendar from these dates. Take later ordinary deadlines from Schedule 17 to the Finance Act 2026 and from current HMRC guidance, and check both at the time you use them.

The penalty table moves with the deadlines

The instrument also makes a temporary adjustment to the failure-to-file penalty table in Schedule 24 to the Finance Act 2021. The adjustment aligns that table with the extended first period and the 2 specified quarterly periods.

That makes deadline ownership an operational control, not a diary exercise. A missed handover from the annual period to the first quarterly periods can affect both filing and payment.

What to do now

Build a deadline register with a rule column, not only a date column. For each entity, record who triggered registration, when the trigger occurred, which accounting period applies, and which source supports the deadline.

Set internal preparation dates ahead of the statutory dates. Label them clearly as internal controls, and keep them distinct from the legal deadlines above.

Assign one named person to monitor the opening of the registration service and any later HMRC guidance.

Then map each UK importing entity to the correct transitional rule, and test whether the required data can actually be closed on that timetable.

This article is based on the as-made text and the official PDF of SI 2026/830. Recheck the instrument, Schedule 17 and current HMRC guidance before you fix a live registration, return or payment deadline.