Transport experts are urging the UK government to exclude American cars from any post-Brexit trade deal owing to safety fears.
There is growing concern that US vehicles do not meet British safety standards for accidents with pedestrians, cyclists and children.
While the number of people killed in road crashes has fallen in the states, the number of pedestrians killed by vehicles had risen by 53% over a decade, according to a study by US Insurance Institute for Highway Safety.
This is due to large SUVs posing a danger to pedestrians, even though they keep occupants safe.
The UK government is expected to include cars in any new transatlantic trade agreement and said safety standards would not be "diminished" as a result of talks.
But the parliamentary advisory committee on transport safety has written to trade secretary Liz Truss, raising concerns that pressure for lower safety standards could be applied in negotiations regarding the automotive sector.
“US vehicle safety standards are much lower than those permitted for vehicles sold in the UK,” warned the committee.
David Ward, president of the Global New Car Assessment Programme, told BBC News: “US crash standards are much lower for pedestrians... we simply can’t let American vehicles into the UK if they don’t meet our standards.”
A Department for Transport spokeswoman said the government would decide its own safety regulations after Brexit.
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“Road safety or environmental standards will not be diminished as part of a free trade agreement with the USA or any other country,” she said.
Matt Blunt, president of the American Automotive Policy Council, which represents US carmakers, disputed the safety claims.
"Cars, SUVs and other light trucks that meet US safety standards achieve equivalent safety performance to the safety standards applied in the European Union,” he told the BBC.
"A US-UK trade agreement should address the tariff and technical barriers to open US-UK automotive trade. This would increase competition and provide more consumer choice."