The lead Brexit envoy in the UK’s embassy in Washington has resigned, saying she can no longer “peddle half-truths” on behalf of political leaders she does not trust.

Alexandra Hall Hall sent a resignation letter earlier this week, warning that increasing demands were being placed on civil servants not to be “fully honest” with the public.

It comes as leaked internal documents show there will be customs checks and controls between Britain and Northern Ireland, despite claims by Boris Johnson that there would not be.

The government has also been forced to issue a series of denials that aspects relating to the NHS would be on the table in US trade talks despite some evidence that meetings are already taking place.

A spokesperson for the Foreign Office confirmed the news, first reported by CNN, and said: “We won’t comment on the detail of an individual’s resignation.”

Biggest lies told by Boris Johnson Show all 5 1 /5 Biggest lies told by Boris Johnson Biggest lies told by Boris Johnson Made-up quote for The Times Johnson was sacked from The Times newspaper in the late 1980s after he fabricated a quote from his godfather, the historian Colin Lucas, for a front-page article about the discovery of Edward II’s Rose Palace. “The trouble was that somewhere in my copy I managed to attribute to Colin the view that Edward II and Piers Gaveston would have been cavorting together in the Rose Palace,” he claimed. Alas, Gaveston was executed 13 years before the palace was built. “It was very nasty,” Mr Johnson added, before attempting to downplay it as nothing more than a schoolboy blunder. PA Biggest lies told by Boris Johnson Sacked from cabinet over cheating lie Michael Howard gave Boris Johnson two new jobs after becoming leader of the Conservatives in 2003 – party vice-chairman and shadow arts minister. He was sacked from both positions in November 2004 after assuring Mr Howard that tabloid reports of his affair with Spectator columnist Petronella Wyatt were false and an “inverted pyramid of piffle”. When the story was found to be true, he refused to resign. PA Biggest lies told by Boris Johnson Broken promise to boss In 1999 Johnson was offered editorship of The Spectator by owner Conrad Black on the condition that he would not stand as an MP while in the post. In 2001 he stood - and was elected - MP for Henley, though Black did allow him to continue as editor despite calling "ineffably duplicitous" PA Biggest lies told by Boris Johnson Misrepresenting the people of Liverpool As editor of The Spectator, he was forced to apologise for an article in the magazine which blamed drunken Liverpool fans for the 1989 Hillsborough disaster and suggested that the people of the city were wallowing in their victim status. “Anyone, journalist or politician, should say sorry to the people of Liverpool – as I do – for misrepresenting what happened at Hillsborough,” he said. PA Biggest lies told by Boris Johnson ‘I didn’t say anything about Turkey’ Johnson claimed in January, that he did not mention Turkey during the EU referendum campaign. In fact, he co-signed a letter stating that “the only way to avoid having common borders with Turkey is to vote Leave and take back control”. The Vote Leave campaign also produced a poster reading: “Turkey (population 76 million) is joining the EU”

In her resignation letter, Ms Hall Hall said the government’s approach had undermined the British diplomats’ position abroad and that for her the situation had become “unbearable personally” and “untenable professionally”.

“I have been increasingly dismayed by the way in which our political leaders have tried to deliver Brexit, with reluctance to address honestly, even with our own citizens, the challenges and trade-offs which Brexit involves; the use of misleading or disingenuous arguments about the implications of the various options before us; and some behaviour towards our institutions, which, were it happening in another country, we would almost certainly as diplomats have received instructions to register our concern,” she wrote in her letter, dated 3 December.

“It makes our job to promote democracy and the rule of law that much harder, if we are not seen to be upholding these core values at home.”

She added: “I am also at a stage in life where I would prefer to do something more rewarding with my time than peddle half-truths on behalf of a government I do not trust.”

Ms Hall Hall was previously Britain’s ambassador to Georgia. She said her departure had nothing to do with being “for or against Brexit, per se”, but frustration with how policy was being implemented.

The government has lost several senior diplomats during Brexit talks, most notably Sir Ivan Rogers, its former ambassador to the EU. Sir Ivan last month accused Mr Johnson of “diplomatic amateurism” over Brexit.