(CNN) A senior British diplomat in the US has quit with a blast at the UK government over Brexit , saying she could no longer "peddle half-truths" on behalf of political leaders she did not "trust."

In a searing resignation letter delivered just over a week before the UK general election , Alexandra Hall Hall, the lead envoy for Brexit in the British Embassy in Washington, said that she had become increasingly dismayed by the demands placed on the British civil service to deliver messages on Brexit which were not "fully honest."

The reluctance of Britain's leaders to play straight with the public on Brexit, Hall Hall said, had undermined the credibility of UK diplomats abroad. Her position had become "unbearable personally" and "untenable professionally," she wrote in the letter, which has been obtained by CNN.

Hall Hall's decision to quit, and her no-holds-barred resignation letter, comes at a moment of deep political sensitivity for UK Prime Minister Boris Johnson, who is seeking re-election on the promise that he can "get Brexit done." Johnson has been ahead in opinion polls, but, as his lead has narrowed in recent days, officials are nervous about anything that could undermine his chances of winning a parliamentary majority for his Conservative party in the election next Thursday.

An official at the Foreign Office, who was not authorized to brief the press, confirmed that Hall Hall had resigned but had not seen her letter. A Foreign Office spokesperson said: "We won't comment on the detail of an individual's resignation." Downing Street referred CNN to the Foreign Office.

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.

Though Hall Hall did not refer to Johnson or any other UK leader by name in her letter, she expressed concern about the divisive rhetoric that has characterized British politics since the Brexit referendum. Johnson's comments have hardened in recent months. He has attacked attempts to prevent a no-deal Brexit as "surrendering" to Brussels and dismissed fears that his language encourages supporters to abuse his opponents. Johnson has defended his rhetoric, telling the BBC after a particularly rancorous parliamentary debate in September that avoiding such terms risked "impoverishing the language and diminishing parliamentary debate".

Much of the blame for the strategy has been pinned on Johnson's lead adviser, Dominic Cummings , who ran the Vote Leave campaign in the Brexit referendum of 2016 and is credited with creating the "Take Back Control" slogan. Cummings has been unrepentant over his tactics.

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.