LONDON, July 15 -- U.S. President Donald Trump's claim that the current Brexit policy would mean Britain could not sign a free trade with the United States was backed by the former top Brexit negotiator on Sunday.

British Prime Minister Theresa May unveiled the details of her Brexit program in a White Paper on Thursday, which contained a willingness to accept European Union (EU) rules on food standards and trading goods.

Trump, on a working visit to Britain, dropped a diplomatic bombshell on the day the White Paper was published, saying in an interview with The Sun newspaper that May's Brexit proposals "would probably end a major trade relationship with the United States", a key benefit Brexiters hoped to gain by leaving the EU.

Although he pulled back on that in a joint press conference with May after scheduled talks on Friday, former Brexit Secretary David Davis, who quit May's government in protest at her Brexit strategy a week ago, said on Sunday that Trump was right.

"Last week, the government's Brexit White Paper told the country it could look forward to a 'common rule book' with the EU. We now face continued harmonisation with the bloc's rules on goods. Unfortunately, this jeopardises the opportunities offered by Brexit," Davis said.

"The chance to become a credible trading partner will be compromised and we will be unable to strike free trade deals. As Donald Trump aptly pointed out, it would 'kill' the prospect of a US-UK deal," Davis wrote in a thought-piece for the Financial Times newspaper website.

Davis further added "without control over goods, (Britain) would lack the crucial leverage to open up the UK's export of services to the rest of the world."

Davis's backing of Trump's dire warning on May's Brexit policy, will further increase pressure on May, who was weakened by Davis's Sunday resignation and further rocked by the resignation of her foreign secretary Boris Johnson on Monday.

Johnson resigned over the same issue. Another two junior ministers and two vice-chairmen of May's Conservative Party also quit over May's Brexit strategy.