A former UK trade chief says Donald Trump is “speaking the truth” when he warns Theresa May’s Brexit deal will wreck hopes of a trade agreement with the US.

Sir Andrew Cahn backed the US president, undermining attempts by a furious Downing Street to fight back against the bombshell dropped in Washington.

Sir Michael Fallon, the former defence secretary – who will vote against a deal he branded “doomed” – also warned that “brushing off” Mr Trump’s embarrassing comments would not work.

No 10 was thrown on the back foot when the president described the Brexit proposals as “a great deal for the EU” and “a very big negative” for the UK’s aim of striking its own trade deals.

Sir Andrew Cahn, the former chief executive of UK Trade & Investment (UKTI), agreed, telling BBC Radio 4 that, unusually, the president “is actually speaking the truth”.

Pointing out any trade talks would not be able to cover key US interests such as “sending their chicken to us, their cars”, he added: “In the end there just wouldn’t be a deal.”

Sir Andrew said the Brexit plan was likely to leave the UK “stuck within the EU customs union for the foreseeable future” – restricting bilateral trade deals to services only.

“The deal Mrs May has come up with doesn’t tell us what the long-term trading arrangements between Britain and the EU would be,” he said.

Cabinet ministers resign over Brexit deal Show all 13 1 /13 Cabinet ministers resign over Brexit deal Cabinet ministers resign over Brexit deal Esther McVey Britain's Work and Pensions Secretary Esther McVey arrives to attend the weekly meeting of the cabinet at 10 Downing Street in London. - Britain's Work and Pensions Secretary Esther McVey resigned from the cabinet over draft Brexit deal AFP/Getty Cabinet ministers resign over Brexit deal Handout image taken from the Twitter feed of @EstherMcVey1 of the resignation letter addressed to Prime Minister Theresa May from Works and Pensions Secretary Esther McVey. PRESS ASSOCIATION Photo. Issue date: Thursday November 15, 2018. Ms McVey has resigned, saying the Brexit deal âdoes not honour the result of the referendumâ. See PA story POLITICS Brexit. Photo credit should read: @DominicRaab/PA Wire NOTE TO EDITORS: This handout photo may only be used in for editorial reporting purposes for the contemporaneous illustration of events, things or the people in the image or facts mentioned in the caption. Reuse of the picture may require further permission from the copyright holder. PA Cabinet ministers resign over Brexit deal Dominic Raab British Brexit Secretary Dominic Raab give a press conference at the end of the final round of talks in Brexit negotiations at the European Commission in Brussels, Belgium EPA Cabinet ministers resign over Brexit deal Handout image taken from the Twitter feed of @DominicRaab of the resignation letter addressed to Prime Minister Theresa May from Brexit Secretary Dominic Raab. PRESS ASSOCIATION Photo. Issue date: Thursday November 15, 2018. Mr Raab has resigned, saying he âcannot in good conscience support the terms proposed for our deal with the EUâ. See PA story POLITICS Brexit. Photo credit should read: @DominicRaab/PA Wire NOTE TO EDITORS: This handout photo may only be used in for editorial reporting purposes for the contemporaneous illustration of events, things or the people in the image or facts mentioned in the caption. Reuse of the picture may require further permission from the copyright holder. PA Cabinet ministers resign over Brexit deal Rehman Chrishti Vice Chairman of the Conservative Party Rehman Chrishti tendered his resignation letter this afternoon PA Cabinet ministers resign over Brexit deal Cabinet ministers resign over Brexit deal Anne-Marie Trevelyan Parliamentary private secretary in the Department of Education Anne-Marie Trevelyan resigned stating that she cannot support the deal Cabinet ministers resign over Brexit deal Handout image taken from the Twitter feed of @annietrev of the resignation letter addressed to Prime Minister Theresa May from Anne-Marie Trevelyan, a parliamentary private secretary in the Department for Education. PRESS ASSOCIATION Photo. Issue date: Thursday November 15, 2018. Ms Trevelyan has resigned saying she cannot support the Brexit deal after negotiations âbuilt on the UK trying to appease the EUâ. See PA story POLITICS Brexit. Photo credit should read: @annietrev/PA Wire NOTE TO EDITORS: This handout photo may only be used in for editorial reporting purposes for the contemporaneous illustration of events, things or the people in the image or facts mentioned in the caption. Reuse of the picture may require further permission from the copyright holder. PA Cabinet ministers resign over Brexit deal Sam Gyimah Universities minister Sam Gyimah resigned on November 30, claiming the government's decision to pull out of the EU's Galileo satellite navigation system as a deciding factor PA Cabinet ministers resign over Brexit deal Shailesh Vara Shailesh Vara who has quit as Minister of State for Northern Ireland, saying he cannot support Theresa May's Brexit agreement, which he said "leaves the UK in a halfway house with no time limit on when we will finally be a sovereign nation" PA Cabinet ministers resign over Brexit deal Cabinet ministers resign over Brexit deal Suella Braverman Brexit minister Suella Braverman has resigned, stating “It is not what the British people, or my constituents, voted for in 2016.” Cabinet ministers resign over Brexit deal Ranil Jayawardena Parliamentary private secretary to the ministry of justice Ranil Jayawardena resigned as he could not back the deal "in good conscience"

“So, after the transition period is finished in two or three or four years, anything could happen.”

Sir Andrew, who advised on setting up the single market in the 1980s, added: “It will be very difficult to do independent trade deals for as long as we can see looking forward.”

The warning came as Sir Michael, who resigned a year ago, called for Brexit to be postponed “for two or three months”, to allow negotiators to go “back to Brussels” for a better deal.

“It’s no use us just brushing that off, saying ‘no, no we can do a deal with America’,” he said.

“He’s the president of the United States, and if he says it’s going to be difficult, then it certainly looks like it’s going to be difficult. This is not a good deal and we need a better deal.”

At the White House on Monday, Mr Trump said: “Sounds like a great deal for the EU. I think we have to take a look at seriously whether or not the UK is allowed to trade” – a reference to signing trade deals.

“Right now, as the deal stands, they may not be able to trade with the US and I don’t think they want that at all. That would be a very big negative for the deal.”

Later, the prime minister, speaking on a visit to Wales, insisted the “political declaration“ agree with the EU would allow the UK to strike its own trade deals.

“It clearly identifies that we will have an independent trade policy and we will be able to do trade deals, to negotiate trade deals, with countries around the rest of the world,” she said.

“And, as regards the United States, we’ve already been talking to them about the sort of agreement we could have with them in the future.

“We’ve got a working group set up which is working very well, has met several times, [and we’re] continuing to work with the US on this.”

And David Lidington, Ms May’s de facto deputy, shrugged off Mr Trump’s comments, saying: “I don’t think it was that unexpected.”