Video report by ITV News Correspondent Angus Walker
Sir Alan Duncan has quit as a minister at the Foreign Office with a fiery letter in which he makes several thinly veiled attacks on Boris Johnson.
His departure comes a day before the winner of the Tory leadership race is announced and is expected to be followed by a wave of resignations anticipating Mr Johnson taking power.
The foreign minister - who has spoken critically of Mr Johnson - handed his letter in on Monday morning as voting by Tory members comes to an end.
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What are the most outspoken lines from the letter?
Sir Alan confirmed he was standing down "in order to be free to express my views" ahead of the change of power.
"I have served with two very different foreign secretaries," he notes pointedly of his time alongside Mr Johnson and Jeremy Hunt, who are contesting the Tory leadership.
Noting his successes and missed opportunities, he points a silent finger at Mr Johnson, whose comments in 2017 over Nazanin Zaghari-Ratcliffe - the British-Iranian woman imprisoned in Tehran - soured efforts to free her.
"I remain deeply upset that some fruitful discussions I had initiated about the possible release of Nazanin Ratcliffe were brought to such an abrupt halt," Sir Alan writes.
He saves his most outspoken criticism for the end of the letter, writing: "It is tragic that just when we could have been the dominant intellectual and political force throughout Europe we have had to spend every day working beneath the dark cloud of Brexit."
And in a final word for Mrs May, he offers: "Your self-esteem can, and forever will, exceed those of your critics."
Sir Alan, who has served as Tory MP for Rutland and Melton since 1992, was appointed Minister of State at the Foreign and Commonwealth Office on 17 July 2016.
Chancellor Philip Hammond and Justice Secretary David Gauke have already confirmed they will quit rather than be sacked by Mr Johnson.
What has Sir Alan Duncan said previously about Boris Johnson?
Sir Alan has been one of Mr Johnson's fiercest critics on the Tory benches.
In June he described Mr Johnson as a "circus act" and last year he promised to end the former foreign secretary's political career over his comparison of Theresa May's Brexit deal to a "suicide vest".
He renewed his criticism of Mr Johnson earlier this month in the wake of the resignation of UK ambassador to the US Sir Kim Darroch.
Sir Alan said his Tory colleague had shown a lack of support to a "supreme diplomat" and "contemptible negligence" for failing to defend him from US criticism.
"Boris Johnson ... has basically thrown our top diplomat under a bus," he said.
What's happening at the end of the Tory leadership race?
Sir Alan's resignation came after Mr Johnson restated his firm intention to get the UK out of the European Union by the end of October, claiming a deal with Brussels could be reached if the country has the "will" and the "drive" for Brexit.
The ballot of Conservative Party members will close at 5pm on Monday, with the result due to be announced on Tuesday morning.
Mrs May will tender her resignation to the Queen after taking Prime Minister's Questions in the Commons on Wednesday afternoon, with the new Tory leader set to enter Number 10 soon after.