In her resignation letter, addressed to deputy ambassador Michael Tatham and which Hall Hall shared widely with colleagues in the diplomatic service, she said that her departure had nothing to do with being "for or against Brexit, per se," but instead was an expression of frustration about how the policy was being carried out.

Hall Hall, a 33-year veteran of the UK foreign service, and a former ambassador to Georgia, said UK institutions had been undermined and the reputation of British democracy abroad had been imperiled.

"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 December 3.

"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."

Hall Hall said she could no longer reconcile her commitment to the job with the demands made of her. "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," she wrote in the letter.

But Hall Hall made clear in her letter that she was not motivated to step down by any personal convictions over the outcome of Brexit, and that she had enthusiastically accepted the position as Brexit Counsellor at the Washington embassy in 2018. "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 in her resignation letter.

Her complaints have echoes of the discontent of some diplomats in the US foreign service under US President Donald Trump. Since he was elected, scores of diplomats have left the State Department and senior roles have been left unfilled. Hall Hall's resignation is a sign that under Johnson, there is similar unease.

As UK Brexit Counsellor, Hall Hall was tasked with explaining Britain's approach to leaving the European Union to US lawmakers and policy makers on Capitol Hill and in the White House. She suggested that her diplomatic role -- intended to be politically neutral -- was co-opted to deliver messages that were "neither fully honest nor politically impartial." Hall Hall said that she had filed a formal complaint about being asked to convey overtly partisan language on Brexit in Washington.

Hall Hall said she was resigning now, rather than after the election, so that her decision could not be portrayed as a reaction to the result. She is expected to leave the embassy next week, and is quitting the diplomatic service completely.

"Each person has to find their own level of comfort with this situation," she wrote in her letter. "Since I have no other element to my job except Brexit, I find my position has become unbearable personally, and untenable professionally."

Attempts by CNN to reach Hall Hall for further comment were unsuccessful.