The US agreed on 8 May to drop import taxes on UK steel as part of a trade deal with the UK, but the original 25% tariff has been kept in place while the details of the deal are worked out.
UK Steel director general Gareth Stace said: "The deal that Prime Minister Sir Keir Starmer and President Donald Trump struck just a few short weeks ago is yet to be finalised, so this doubling of tariffs plunges the UK steel industry further into confusion...it is yet another body blow for all UK steelmakers in this torrid time.
"UK steel companies are this morning fearful that orders will now be cancelled, some of which are likely being shipped across the Atlantic as we speak."
Mr Stace said the trade group would now be "pressing our government to finalise the agreement to eliminate UK steel import tax and for it to come into effect urgently".
"UK steelmakers should not have to shell out for this new steep hike in US steel tariffs - all we want is to continue producing the steel our US customers value so highly," he said.
A spokesperson for the UK government said: "The UK was the first country to secure a trade deal with the US earlier this month and we remain committed to protecting British business and jobs across key sectors, including steel."
The Guardian reported on Saturday that UK business secretary Jonathan Reynolds will meet his US counterpart Jamieson Greer at an OECD meeting - a global policy forum - in Paris next week, where they will seek to agree a timeline for exempting the UK from the US steel tariffs.