Video report by ITV News Correspondent Angus Walker
Chancellor Philip Hammond has become the second Cabinet minister to announce they will quit the Government if Boris Johnson becomes prime minister.
Mr Hammond, who has been in Number 11 for three years, has been a vocal critic of a no-deal Brexit – something the Tory leadership frontrunner has vowed to do if he cannot secure changes to the Withdrawal Agreement.
Asked on the BBC One’s Andrew Marr show if he thought he would be sacked, Mr Hammond said: “No, I’m sure I’m not going to be sacked because I’m going to resign before we get to that point.
“Assuming that Boris Johnson becomes the next prime minister, I understand that his conditions for serving in his government would include accepting a no-deal exit on the 31st October, and it’s not something that I could ever sign up to.
“It’s very important that the prime minister is able to have a chancellor who is closely aligned with him in terms of policy, and I therefore intend to resign to Theresa May before she goes to the Palace to tender her own resignation on Wednesday.”
ITV News Correspondent Angus Walker on the significance of Philip Hammond's announcement
It comes after Justice Secretary David Gauke – a former Remainer – told the Sunday Times he would also quit on Wednesday if Mr Johnson enters Number 10.
Mr Gauke said: “Given that I’ve been in the Cabinet since Theresa May came to power, I think the appropriate thing is for me to resign to her.”
Mr Hammond - who is dubbed Spreadsheet Philip due to his dry image - had been widely expected to leave office when Mrs May steps down on Wednesday.
Last week, he put down a marker about his willingness to cause trouble on the backbenches for the next prime minister as he and three other Cabinet ministers backed a measure aimed at preventing Mr Johnson suspending Parliament in order to force through a no-deal Brexit.
The Chancellor, Mr Gauke, Business Secretary Greg Clark and International Development Secretary Rory Stewart did not support the Government in the vote.
He has previously promised to do "everything" in his power to block a no-deal Brexit and last week left open the possibility of voting to bring down a Conservative government led by Mr Johnson if the UK was on course to crash out of the EU without a deal on October 31.
Asked on Marr whether he would vote against the new prime minister in a vote of no confidence, Mr Hammond said: "I don't think it will get to that and, while many clever people have been scratching their heads, Parliamentary process is extraordinarily complex and sometimes arcane.
"I am confident that Parliament does have a way of preventing a no-deal exit on October 31 without parliamentary consent and I intend to work with others to ensure parliament uses its power to make sure that the new Government can't do that.
"The point of that is not to inflict some defeat on the new government, it is to ensure that the new government focuses then on trying to achieve a sensible, negotiated settlement with the EU that protects our economy and allows us all to get on with our lives."
Earlier, Defence Minister Tobias Ellwood repeatedly sidestepped questions over whether he would serve in government led by Mr Johnson.
"I really get frustrated with this energy towards no-deal. I know all my parliamentary colleagues on all sides of the House recognise the dangers of no-deal," he told Sky's Sophy Ridge on Sunday when asked.
"The fact that we keep talking about it - it isn't a solution."
Over the past few weeks, ballot papers have been sent out to the 160,000 Conservative Party members who will decide whether the Tories' new leader, and therefore prime minister, will be Boris Johnson or Foreign Secretary Jeremy Hunt.
The winner of the Tory leadership race will be announced on Tuesday.
On Wednesday, Theresa May will speak in the Commons at her last Prime Minister's Questions, before heading to Buckingham Palace to resign her position to the Queen.