The US President has set out plans for steel imports to face a 25% tariff and aluminium 10%.
Richard Warren, head of policy at UK Steel, said the US was a significant export market for British producers.
“It would have a significant impact if it did go ahead,” he told BBC Radio 4’s Today programme.
Mr Trump said US steel and aluminium had been “decimated” by “unfair” trade policies, and insisted he would not let America be “taken advantage of any longer”.
Further details on the plans are expected next week.
Mr Warren said the US was a “fairly significant” market, accounting for around 15% of UK steel exports.
“At the minute we don’t know exactly how these tariffs will be implemented, so we’ll have to wait until the official announcement next week,” he said.
“But the assumption, from what he said, is that it would be a blanket tariff of 25% on all steel products.
“It’s very big, considering the thin margins that larger steel operators will be operating to - 25% is a significant tariff.”
The European Union has indicated it could retaliate, potentially starting a trade war with the US.
European Commission president Jean-Claude Juncker said: “We will not sit idly while our industry is hit with unfair measures that put thousands of European jobs at risk.
“I had the occasion to say that the EU would react adequately and that’s what we will do.
“The EU will react firmly and commensurately to defend our interests. The Commission will bring forward in the next few days a proposal for WTO-compatible counter-measures against the US to rebalance the situation.”
Unite union national officer for steel Tony Brady said: “US tariffs on UK steel would be devastating for the British steel industry and the thousands of workers who have battled for its survival alongside their trade unions.
“Any tariffs imposed on UK steel by President Trump on a scale that is being mooted would be misguided and deprive US manufacturers of some of the most specialist steel in the world.
“The dumping of cheap Chinese steel into the UK took our world-class British steel industry to the precipice because of the Government’s inaction.
“Government ministers and Theresa May must back Britain’s steelworkers and manufacturing communities by securing assurances from President Trump that they will not be caught up in a global tariff war between the US and countries such as China.”