Ministers are facing new questions about the effectiveness of Britain's automotive testing regime in the wake of the Volkswagen emissions scandal.
Former Liberal Democrat transport minister Norman Baker has claimed David Cameron delayed imposing new emission limits following a personal request from German Chancellor Angela Merkel in order to protect the German motor industry.
Mr Baker, who was transport minister in the coalition from 2010 to 2013, said the move provoked furious protests from other ministers and from British manufacturers.
The chairman of the Commons Transport Committee, Louise Ellman, said Mr Baker's claims raised serious questions over the effectiveness and the independence of the whole testing regime.
Transport Secretary Patrick McLoughlin announced on Thursday that diesel cars in the UK would be re-tested in the wake of the disclosure that Volkswagen installed sophisticated "defeat" technology in some of their vehicles to fool emissions testers.
Mr Baker told BBC2's Newsnight that efforts to curb harmful emissions had been undermined by Mr Cameron's insistence on deferring to Mrs Merkel.
"Angela Merkel rang the Prime Minister and asked him effectively to defer the arrangements that had been carefully negotiated. He agreed to that, idiotically, and got something inconsequential in return," he said.
"That could have unpicked the whole thing. What we saw then was Cabinet ministers writing formally to the Prime Minister and to the Foreign Secretary to complain about that behaviour.
"The motor industry at the time was furious, The motor industry was saying to me 'Is the prime minister on our side or the side of German manufacturers?'"
Mrs Ellman said his comments raised a "very big question" about outside influences on what should be matters of public safety.
"There is certainly a major question about the influence of the motor industry against the interests of the public," she told the BBC Radio 4 Today programme.
"The whole issue of independence I think is the key issue here. Testing should be independent of manufacturers and of government and it doesn't seem that that has been the case."