The US has delayed its removal of trade tariffs on UK steel and aluminium products amid concerns at the government's approach to the dispute with the EU on post-Brexit trade rules affecting Northern Ireland.
The move was met with a defiant response from Whitehall as a spokesperson said it would not "affect the UK's approach" to the talks with Brussels on changes to the Northern Ireland Protocol - designed to prevent a hard border on the island of Ireland.
Labour described the US move as "unacceptable" while the steel industry urged the British and American governments to "train every sinew" to reach a compromise on the tariffs.
The UK has threatened to trigger Article 16 - an emergency brake that allows the UK or EU to seek to suspend parts of the Brexit agreement that introduced some checks on the movement of goods to Northern Ireland from mainland Britain - if they lead to persistent trade difficulties.
Talks between the EU and Brexit minister Lord Frost are continuing, however European Commission vice-president Maros Sefcovic has expressed frustration this week over a lack of UK compromise.
News of the US intervention came last night when the Financial Times was handed a communication from a US Commerce Department official which suggested that talks with the UK on easing metals tariffs could not move ahead.
The penalties were imposed on the EU by the administration of former president Donald Trump in 2018 and withdrawn by President Biden in October.
But the 25% and 10% tariffs on steel and aluminium imports are set to remain in place for Britain beyond January.
The official, the newspaper said, cited US concerns about British threats to trigger Article 16 especially from the US Congress.