In her resignation letter, seen by US broadcaster CNN, Alexandra Hall Hall said she could no longer "peddle half-truths" on behalf of political leaders she did not "trust".

She said she has become "dismayed" by the reluctance of politicians to "honestly" address the "challenges and trade-offs" involved in leaving the EU.

The Foreign Office said it would not comment on details of her resignation.

However, it did confirm Ms Hall Hall had resigned as UK Brexit Counsellor at the British embassy in Washington - a post which involves explaining the UK Brexit policy to US lawmakers and policymakers.

In her letter, dated 3 December, she wrote: "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."

She also criticised the use of "misleading or disingenuous arguments" and "some behaviour towards our institutions" by politicians, adding that "were it happening in another country, we would almost certainly as diplomats have received instructions to register our concern".

Ms Hall Hall added: "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."

BBC diplomatic correspondent Paul Adams described her letter as "stunningly blunt".

'Unbearable'

Ms Hall Hall, who is a former ambassador to Georgia and has worked in the diplomatic service for 33 years, did not name any specific politicians in the letter, but took aim at the current Conservative government.

She wrote: "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."

When the BBC put Ms Hall Hall's comments to Foreign Secretary Dominic Raab on Friday evening, he said: "I'm not going to talk about employment issues in the civil service."

Diplomats are supposed to be politically neutral and Ms Hall Hall stressed her decision to resign was not tied to her personal views on leaving the EU.

"I took this position with a sincere commitment, indeed passion, to do my part, to the very best of my abilities, to help achieve a successful outcome on Brexit," she wrote, but added her position had become "unbearable personally and untenable professionally".

With a week to go until the UK heads to the polls, Ms Hall Hall insisted she had stood down before the election to avoid her resignation being portrayed as a reaction to its outcome.

CNN reported that she had also filed a formal complaint about being asked to convey overtly partisan language on Brexit.

Ms Hall Hall suggested her role as a diplomat had been diverted to convey messages that were "neither fully honest nor politically impartial."

The UK has been without an ambassador to the US since Sir Kim Darroch resigned in the summer over a row about leaked emails critical of President Trump's administration.