The UK steel industry warned it could be badly hit after US President Donald Trump confirmed plans for a 25% tariff on imports which trade partners have warned could provoke a "global trade war".
The tariff, due to take effect in 15 days, was aimed at protecting national security and American jobs, Mr Trump said.
But businesses say the tariff on imported steel will increase costs, raising prices for consumers and unions have warned it could potentially put people out of work.
UK Steel's director Gareth Stace said the tariffs "would have a profound and detrimental impact on the UK steel sector, which exported some 350,000 tonnes of products to the US in 2017, over 7% of its total exports".
He told ITV News: "We're extremely disappointed that whilst President Trump had loaded the gun, unfortunately last night he actually pulled that trigger which will really impact us, in terms of the British steel sector, of all of our imports to the US which is actually 10% of our total exports worth £360m.
"It will be severely hit and severely affected."
Roy Rickhuss, general secretary of steelworkers' union Community, said: "Donald Trump's short-sighted strategy will put jobs at risk on both sides of the Atlantic.
"It is now more important than ever that Theresa May and Liam Fox use every bit of influence they have left in America to protect the jobs of British steelworkers.
"The steel crisis cost our industry thousands of jobs and the last thing we need now is a global trade war.
"The UK government must work with other EU nations to ensure we do not suffer the global consequences of Donald Trump's actions."
Speaking in the White House with a group of workers, Mr Trump confirmed the levy on steel imports and a 10% tariff on aluminium in response to an "assault on our country" from cheap overseas metal.
International Trade Secretary Liam Fox said it would be "absurd" for the UK to be hit by the tariffs adding that it was "the wrong way" to tackle the problem of cheap steel being dumped on the US market.
There would be exemptions for North America Free Trade Agreement (Nafta) states Mexico and Canada, and Mr Trump said other countries may be able to negotiate to avoid the tariffs.
Mr Trump said if his goals could be accomplished "by other means" the US will remain "open to modifying or removing the tariffs for individual nations as long as we can agree on a way to ensure that their products no longer threaten our security". A spokesperson from the Department for International Trade said: "The Government has been clear that tariffs are not the right way to address the global problem of overcapacity, which requires a multilateral solution.
"We will work with EU partners to consider the scope for exemptions outlined yesterday and continue to work with all the sectors involved in this decision to robustly support our industries and demonstrate the importance of their high-quality products for US industries and security."
On BBC's Question Time, Dr Fox said he would visit Washington next week.
In comments before the tariffs were confirmed, he said the UK produces "very high value steel, some of which can't be sourced in the United States and will simply push up the price of steel there".
"We also make steel for the American military programme so it's doubly absurd that we should then be caught on an investigation on national security.
"So our view is yes we can deal multilaterally with the overproduction of steel, but this is the wrong way to go about it."
Labour MP Stephen Kinnock, whose constituency in south Wales includes the giant Port Talbot steelworks, said Theresa May had "let down" British workers by failing to stand up to the US President.
Aberavon MP Mr Kinnock said: "Theresa May has, again, let down British steelworkers.
"Ever since Trump first announced punitive tariffs on steel and aluminium last week, the British steel industry, unions and steel MPs have been calling on her to make the case against these tariffs and for a UK exemption.
"Tonight we learnt that she has abjectly failed in that task."
The EU's trade commissioner Cecilia Malmstrom said the bloc should be excluded from the measures and she would meet US trade representative Robert Lighthizer to discuss the situation on Saturday.
"We hope that we can get confirmation that the EU is excluded from this," Ms Malmstroem said.
But she warned that if the issue can't be resolved bilaterally or through the World Trade Organization, then "we will have to protect our industry with rebalancing measures."
The EU has warned that it stands ready to slap retaliatory tariffs on U.S. steel, agricultural and other products, like peanut butter, cranberries and orange juice.
"Everything you have for breakfast," Malmstroem said. She noted that under WTO rules, the EU has 90 days to enact the measures.
The head of EUROFER, Europe's main steel federation, said Trump's reasons for slapping tariffs on steel and aluminum were an absurdity and that the move could cost tens of thousands of jobs across the continent.
EUROFER chief Axel Eggert said "the loss of exports to the U.S., combined with an expected massive import surge in the EU could cost tens of thousands of jobs in the EU steel industry and related sectors."
China's government says it "firmly opposes" the tariff increase but has given no indication whether Beijing might impose its own measures in response.
A Commerce Ministry statement on Friday accused the US president of damaging the global trading system by taking unilateral action instead of filing a complaint through the World Trade Organisation (WTO).
A ministry statement said: "The misuse of the 'national security exception' clause by the United States is wanton destruction of the multilateral trade system represented by the WTO and will surely have a serious impact on the normal international trade order. China firmly opposes it."
Beijing has said it was ready to retaliate in the event Chinese companies are hurt but the statement gave no indication of official action.