Northern Ireland is to be left out of the UK’s green car emission restrictions from 1 January, resulting in “Great Britain-only” regulations for carmakers.

A quirk in the regulations means that cars sold in Northern Ireland will count as EU sales for strict new carbon dioxide emission limits.

Under the Northern Ireland protocol, the UK and the EU have agreed a special status in an effort to avoid a border between Northern Ireland and the Republic of Ireland.

Indications that Northern Ireland is being treated differently from England, Scotland and Wales are politically sensitive because of the support of many prominent unionist politicians for Brexit. However, the protocol means that many EU regulations will remain in place in Northern Ireland after 1 January, regardless of the outcome of trade negotiations between the UK and the EU. Those regulations include those limiting CO2 emissions from cars.

All major carmakers who sell cars in the EU must reduce their average carbon emissions to below 95g per kilometre over the course of 2020 and 2021, and the UK has pledged rules that are at least as stringent. Carmakers who do not meet individual targets face fines worth potentially hundreds of millions of euros, with equivalent penalties in sterling in the UK.

A government consultation published this month revealed that Northern Ireland will count as a member of the EU for the purposes of working out average emissions, to avoid the possibility of a car being double-counted.

The consultation paper said: “Newly registered vehicles in Northern Ireland from 1 January 2021 will continue to be the subject of the EU regulation. The retained version of the regulation, and the subject of this consultation, will apply in Great Britain only.”

The effect of the new regulations on businesses and consumers is highly uncertain, given potential variation in consumer demand for different models and individual carmakers’ different circumstances, but the Society of Motor Manufacturers and Traders, the UK car lobby group, has previously warned that carmakers may pull larger cars from UK sale to avoid fines.

The Department for Transport declined to comment.