Foreign Office minister Sir Alan Duncan has announced his resignation, in the first of what is expected to be a stream of ministers walking out of government because they are unwilling to serve under Boris Johnson.

In a resignation letter to Theresa May, the pro-EU minister said it was "tragic" that "the dark cloud of Brexit" was getting in the way of UK influence in the world.

Chancellor Philip Hammond and justice secretary David Gauke have already announced their intention to hand in their resignations on Wednesday, before Ms May goes to Buckingham Palace to hand over the reins of power to a new prime minister.

International development secretary Rory Stewart is also expected to quit, having joined Hammond and Gauke in abstaining in a key Brexit vote last week, though reports suggest business secretary Greg Clark - who also abstained - may stay on.

Duncan has been a fierce critic of Johnson, accusing him of throwing the UK's ambassador in Washington Sir Kim Darroch "under a bus" by failing to back him over the leak of his diplomatic cables because he wanted to "suck up to the President of the United States".

He did not mention Mr Johnson in his resignation letter to Ms May, but took a swipe at his former boss by saying he was "deeply upset" at the failure of discussions to secure the release from an Iranian jail of British-Iranian mother Nazanin Zaghari-Ratcliffe. Nazanin's husband Richard Ratcliffe has said Johnson's inaccurate comments about his wife have made her case more difficult.

In a barb apparently directed at the likely new PM and the hardline Brexiteers who backed him, Duncan told Ms May that he was "sorry" her stint as PM was coming to an end, adding: "You deserved better, but please take lasting comfort from the knowledge that your self-esteem can, and will forever, far exceed that of your critics."

And he 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."

Ms May's official spokesman told reporters: "The PM thanks him for his service and will be replying to his letter later on."

Duncan quit on the day when ballots close in the race to replace Ms May as Conservative leader, with Mr Johnson expected to secure a comfortable victory over rival Jeremy Hunt when results are announced on Tuesday.

(Reuters)

The new PM will start appointing ministers to his cabinet after entering 10 Downing Street on Wednesday afternoon.

Mr Johnson has made clear all members of his cabinet must be ready to accept a no-deal Brexit in October, and a number of ministers who are opposed to leaving the EU without an agreement are expected to walk out rather than wait to be sacked.

Sir Alan served as Mr Johnson's deputy in the Foreign Office for two years until the Brexit figurehead's resignation over Brexit a year ago.

He backed Mr Hunt for the leadership, branding Johnson a "circus act" for his gaffe-prone record as foreign secretary.

Speaking last month, the Rutland and Melton MP said: “Some of his moments as foreign secretary were diplomatically awkward… which takes us to the serious points of the difference between the two candidates who are trying to become prime minister.

Anti-Brexit protestors flew an effigy of Boris Johnson over Westminster at the weekend (Rex Features) (Rex)

“You can have a circus act, or you can have a really serious person, who does do detail and is respected in other countries, and who will be taken seriously in discussions on whatever topic, Brexit or something else.”

Another Hunt-backing Conservative MP, Greg Hands, said it was "absurd" to walk out ahead of the announcement of the new leader.

"In my view, pre-emptive ministerial resignations ... in case your own democratically-elected party leader is not to your liking are absurd," said Mr Hands.

"And I say that as a committed Jeremy Hunt supporter. Such moves make a Corbyn government one step more likely."

Sir Alan served in the shadow cabinets of Michael Howard and David Cameron, but was not appointed to the coalition cabinet in 2010, instead accepting a lower-ranking ministerial role in the Department for International Development, where he served until 2014. He returned to the political front line as Foriegn Office minister for Europe and the Americas when Ms May took office in 2016.

Responding to his resignation, the CEO of the Best for Britain campaign for a second referendum, Naomi Smith, said: "Sir Alan's resignation shows what a disaster we're heading for under Boris Johnson. He will be joined by other key Government figures, all of whom believe there's nothing patriotic about serving under a prime minister willing to wreck the economy for Brexit.