The British diplomat in charge of explaining Brexit to the US government, Congress and public, has resigned, saying she was no longer prepared to “peddle half-truths on behalf of a government I do not trust”.

Alexandra Hall Hall, the Brexit counsellor at the UK embassy in Washington, had been frustrated with the job for some time, according to friends and colleagues.

They said she felt she was not being given enough reliable information to do her job, which was to explain Britain’s departure from the EU to US audiences and help promote a strong US-UK relationship post-Brexit.

Her resignation, which was addressed to the chargé d’affaires, Michael Tatham, and circulated among close colleagues at the embassy, was damning in its description of the Johnson government’s integrity.

“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,” Hall Hall wrote in the letter, dated 3 December.

The letter was published by CNN, and its authenticity was confirmed to the Guardian by diplomatic sources.

“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. “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.”

Hall Hall has 33 years of experience as a diplomat. She was the UK’s ambassador to Georgia for three years until 2016, and was head of the Foreign Office human rights department. She also served in Bangkok, New Delhi and Bogotá. She was seconded to the US state department from 2002 to 2004, working on promotion of human rights and democracy in the Middle East.

She was still working at the embassy on Friday, but is due to leave in the week before Christmas. She is married to the American head of the International Republican Institute, Daniel Twining.

“She was really good at her job and really well respected around town,” said a source who had frequent dealings with Hall Hall over Brexit and its consequences. “She has done everything right, and has had legitimate concerns. This is not something that has just developed in the past week or two. It dates back, but I had the impression her concerns were being dealt with internally. I am surprised it has come to this.”

A foreign office spokesperson said: “We don’t comment on the detail of an individual’s resignation.”

After her posting in Tbilisi, Hall Hall had taken a break from government, moving to Washington to become a senior fellow at the Atlantic Council, where her writing focused on the need for western solidarity in the face of Vladimir Putin’s expansionism. But last year, she took the Brexit counsellor job at the embassy.

“She was a Remainer, but she told me that at the end of the day she supported her country and her government and she saw the role as crucial to making a success of the transition,” a former colleague recalled.

Daniel Fried, a former US assistant secretary of state for Europe, said: “Ambassador Hall Hall is known to her US colleagues, myself included, as a person of integrity and insight. I cannot comment on the specifics of her charges, but she is a credible and serious person and as such her words carry weight.”

Nile Gardiner, director for the Margaret Thatcher Centre for Freedom at the Heritage Foundation in Washington, said: “It is very, very disappointing to see a British diplomat not implementing the official policy of their government.”

“Diplomats should not be giving their own interpretation of information provided by their own government,” Gardiner added.

The UK embassy has been without an ambassador since July, when Kim Darroch resigned following the leak of internal foreign office cables in which he described the Trump administration as dysfunctional, faction-riven and inept. The nomination of a successor has been delayed by political turmoil in Britain.