Theresa May has told President Donald Trump in a phone call that US tariffs on EU steel are "unjustified and deeply disappointing", Downing Street has said.
The Prime Minister's official spokesman characterised the 30-minute call as "constructive" and said they agreed to discuss the issue further at the G7 summit in Canada later this week.
International Trade Secretary Liam Fox, speaking in the House of Commons, told MPs he was "disappointed" with the tariffs, but added that he hoped once the UK had left the EU "we'll have no problems with a UK exemption".
Dr Fox welcomed the European Union's decision to take the matter to the World Trade Organisation, but raised fears that it could be difficult for the WTO to act.
He said: "The problem with using national security, as has been done in this case, is twofold.
"Were the United States to be successful it sets a precedent for others do the same and to use national security as pretext for protectionism, and secondly, it leads the WTO into the realms of having to determine what is and what is not acceptable as a definition for national security, something the WTO has always shied away from."
Addressing the EU's decision to launch counter-tariffs on a variety of US products towards the end of June, Dr Fox refused to say whether the UK would back them.
He said: "We still want to see what the measures themselves are, specifically we have been talking to the Irish government about the issue of bourbon being on the list because of the potential implications for the Scotch whisky industry and the Irish whiskey industry."
Gareth Stace, director of trade body UK Steel, said the US tariffs were a "hammer blow" to British producers which will "significantly" undermine their ability to export to the States.
"This cannot stand," said Mr Stace. "Liam Fox has today shown that he stands with the UK steel sector in condemning the 'unjustified' US steel tariffs, supporting a strong and swift EU response, and using our close relationship with the US to exert influence where possible.
"The top priority for UK Government must be to continue to support the European Commission to secure a complete and permanent exemption for the EU from these absurd tariffs."