Community and Unite unions fear Donald Trump steel tariff
There are calls for the UK government and European Union to take action to protect the steel industry from the "devastating" impact of a 25% US tariff on imports.
While the details are to be announced, there are fears such a move could damage the UK steel industry's fragile recovery from the crisis of 2016.
Unite and Community unions have also warned that jobs would be at risk.
The US accounts for around 10% of Tata Steel Europe's sales.
Tata welcomed the EU's announcement "that appropriate and swift measures will be taken to safeguard the interests and jobs in our industry".
The US government is expected to impose the 25% tariff on steel imported from any country.
It will offer some protection to US steel companies at a time when more steel is being produced globally than there is demand for.
The move by President Donald Trump - who is worried that the US is "losing billions of dollars on trade"- differs from the Obama administration's tariffs on Chinese steel.
That was a measure to prevent what was perceived as the dumping of products below what it cost to produce them.
Tata's Port Talbot site, the UK's biggest steelworks, was put up for sale in 2016 along with the rest of the company's UK operations.
Sites in the north of England and Scotland were bought by Liberty Steel and British Steel.
The company is now in merger talks with Thyssenkrupp about the rest of its UK business, including its Welsh sites.
The Bush administration introduced steel tariffs in 2002 which lasted for 20 months.
It prompted threats of retaliation from the European Union on US products before the policy was abandoned.
The tariffs will have an impact on Tata, Liberty and British products made across the UK.
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Roy Rickhuss, general secretary of steelworkers' union, Community, said: "This news is deeply worrying for the UK steel industry.
"Donald Trump is putting jobs at risk on both sides of the Atlantic. Thousands of steelworkers across the country voted for Brexit on the promise it would deliver a new era of international trade.
"The prime minister talks of a 'special relationship' with the USA and we now need to work together to ensure the high quality steel products that the UK exports to the USA are exempt from this tariff."
It urged the UK government to seek assurances from President Trump that they will not be caught up in a global tariff war between the US and countries such as China.
Aberavon MP Stephen Kinnock said: "This reckless and impetuous announcement from Donald Trump is deeply worrying for the British steel industry."
The Tata Steel spokesman added: "The EU must not allow the moderate recovery in our industry over the last year to be destroyed by the EU's most important political ally."