HM Revenue & Customs (HMRC) updated its official guidance on 3 November, clarifying how importers must record and retain the net mass of goods covered by the UK Carbon Border Adjustment Mechanism (CBAM). The mechanism is due to begin on 1 January 2027, affecting specified imports into Britain of aluminium, cement, fertiliser, iron and steel, and hydrogen.
Importers must record the goods’ net mass in kilograms, excluding packaging and reflecting their weight at the time of import. The guidance allows weights to be entered to six decimal places. For net masses above 1 kg, fractional kilograms may be rounded to the nearest whole kilogram; quantities below 1 kg must instead retain decimal precision and cannot be rounded to zero or one kilogram.
UK CBAM calculations depend on the quantity of covered imports and their relevant emissions intensity. Differences between weights on invoices, customs declarations, transport documents and supplier emissions reports could make tax calculations and supporting evidence more difficult to reconcile. The revised guidance addresses recordkeeping precision and does not introduce a new carbon tax.
Evidence and data across the supply chain
Importers must retain records of commodity classifications, import dates, values, origin and applicable exemptions. Supporting documentation generally has to be kept for six years under HMRC’s specified retention rules. Importers reporting actual emissions must also retain the relevant verification report or summary; those without acceptable verified emissions evidence may have to use regulatory default values.
The start of the mechanism and the registration timetable are separate: HMRC’s published guidance says registration opens on 1 January 2028, a year after the CBAM is due to take effect. The requirements make transaction-level customs accuracy important alongside verified emissions information for UK carbon-border compliance.
For Serbian, Montenegrin and other Southeast European suppliers, the guidance affects data exchanges with British industrial customers. Exporters are advised to reconcile packing lists, commercial invoices, product classifications and emissions records before shipment. Steel suppliers selling different grades, shapes or fabricated products should keep weights and product codes consistent across manufacturing and customs systems.
Banks providing trade finance may need to assess whether customers can reliably calculate product quantities, carbon costs and exemptions before financing sizeable import transactions. Those calculations rely on accurate import data as well as emissions information, linking customs records and supporting evidence across the transaction.