A prominent critic of Boris Johnson has quit as a Government minister ahead of the result of the Tory leadership race, warning that Brexit had cast a "dark cloud" over Britain.
Sir Alan Duncan resigned from the Foreign Office before the new prime minister takes office.
Mr Johnson is widely expected to defeat Foreign Secretary Jeremy Hunt to become the Tory leader when the result of the contest is announced on Tuesday.
In his resignation letter, Sir Alan said he had "served with two very different Foreign Secretaries" - Mr Johnson and Mr Hunt.
Sir Alan said: "The UK does so much good in the world.
"It is tragic that just when we could have been the dominant intellectual and political force throughout Europe, and beyond, we have had to spend every day working beneath the dark cloud of Brexit."
His resignation will not be the last, with Chancellor Philip Hammond and Justice Secretary David Gauke having already confirmed they will quit rather than be sacked by Mr Johnson.
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.
On the eve of the leadership ballot closing, Mr Johnson - who is widely expected to become prime minister on Wednesday - said it was time the country rediscovered its "sense of mission".
He wrote in the Daily Telegraph: "If they could use hand-knitted computer code to make a frictionless re-entry to Earth's atmosphere in 1969, we can solve the problem of frictionless trade at the Northern Irish border.
"It is time this country recovered some of its can-do spirit. We can come out of the EU on October 31, and yes, we certainly have the technology to do so. What we need now is the will and the drive."
The ballot of Conservative Party members will close at 5pm on Monday local time 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.
Australian Associated Press