Fears are growing about the prospect of a trade war after Donald Trump agreed to slap hefty tariffs on steel and aluminium imports into the US.

The US president said steel products face a 25% tariff, and aluminium goods 10% because the US was victim to "unfair trade". The US imports steel from more than 100 nations.

A British union source told the Telegraph that the move could have a "devastating" impact on the UK industry after it emerged that senior officials had tried to dissuade the US administration from the biggest protectionist move since he became president.

An official in the UK's Business, Energy and Industrial Strategy Department said Britain's annual exports to the US worth £360m would be harmed.

Cabinet ministers such as Greg Clark, the Business Secretary, and Liam Fox, the International Trade Secretary, had made private appeals against the move, the Telegraph reported.

Clark had lobbied the US against imposing the tariffs four times since November. Senior UK figures trying to dissuade Trump had told The Daily Telegraph they were "deeply concerned" by the decision.

A UK government spokesman said before the decision: "We have made very clear to the US that we are deeply concerned by any measures that would affect the UK steel and aluminium industries.

While still light on specifics, Trump tweeted that the tariffs would "for a long period of time". It is not clear if it would just apply to steel from China and India, but concerns are growing that other countries could put in place tariffs in retaliation.

Trump tweeted: "Our Steel and Aluminum industries (and many others) have been decimated by decades of unfair trade and bad policy with countries from around the world.

"We must not let our country, companies and workers be taken advantage of any longer. We want free, fair and SMART TRADE!"

A British trade union source said: "It will be devastating for British steel."

The Telegraph reported that the president had been given the option of a 24% global tariff on all steel imports, a 53% tariff on 12 countries including China and India but not the UK, or a quota of all steel products equal to 63% of each country's 2017 exports to the US.

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US president Donald Trump