UK CBAM first deadlines: 31 January 2028 and 31 May 2028

Your first UK CBAM deadline is probably not the date already in your compliance calendar. SI 2026/830 sets a transitional calendar for registration, returns and payment in the opening periods of the regime.

The instrument is short, so 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.

How the transitional calendar works

The instrument replaces some ordinary deadlines for the opening periods only. It changes three things: one registration deadlineregistration deadlineThe last date by which a person liable to register must complete UK CBAM registration., the length of the first accounting periodaccounting periodThe period for which a UK CBAM return and payment are calculated., and two quarterly return deadlines.

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 deadline.

The 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.

Show this distinction in your internal checklist. A generic line such as "registration due by January" misstates the rule for anyone outside that transitional group.

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

The first accounting period runs for a full year

The first accounting period covers 1 January to 31 December 2027. Both the return and the payment for that period are due by 31 May 2028.

The first period therefore runs for one full year, so do not model it as 4 ordinary quarterly returns because quarterly periods appear later in the regime.

31 May 2028 is a filing date, not the date on which data collection starts. You prepare the return after the year closes. For that, you need transaction, origin, weight, emissions and relief evidence from the whole of 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, it is 29 September 2028.

These 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 when you use them.

The penalty table moves with the deadlines

The instrument also temporarily adjusts 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.

Deadline ownership is therefore an operational control, not a diary entry. A missed handover from the annual period to the first quarterly periods can affect both filing and payment.

What it means for importers

Your first UK CBAM returnUK CBAM returnThe self-assessed HMRC filing for a UK CBAM accounting period. looks back over a full year of imports. Align the 2027 import ledger, the CBAM data trail and the period-end review well before 31 December 2027.

What to do

• 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 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.
• Map each UK importing entity to the correct transitional rule. Then test whether you can close the required data on that timetable.

Stuck on this in your own import data? Write to us and we will look at it with you.

This article explains the transitional deadlines in the as-made text and official PDF of SI 2026/830. Recheck the instrument, Schedule 17 and current HMRC guidance before you fix a live deadline. It is not legal, tax or customs advice.