Chancellor of the Exchequer Philip Hammond has said that he will resign after Prime Minister's Question Time on Wednesday, before the next prime minister has a chance to fire him, the BBC has reported.
"I am sure I am not going to be sacked because I am going to resign before we get to that point," Hammond said.
"Assuming that Boris Johnson becomes prime minister, I understand that his conditions for serving in his government would include accepting a no-deal (EU) exit on the 31st of October. That is not something I could ever sign up to," he added.
Hammond promised to campaign to try to stop a potential no-deal Brexit from taking place without parliamentary consent, and said that he would "strongly support" a future Conservative government that can reach a Brexit deal with Brussels.
The chancellor said he was certain that parliament could still find a way to stop a no-deal Brexit.
Last week, Hammond said he would "not exclude anything" in his efforts to stop a no-deal Brexit, including backing a no-confidence motion against the government if a future prime minister tried to suspend parliament to move forward with Brexit. According to Hammond, another Brexit extension was "absolutely necessary," since there isn't enough time for London to renegotiate with the EU ahead of the 31 October deadline.
On Thursday, Hammond, Secretary of State for Justice David Gauke, and Minister of State for International Development Rory Stewart have announced their readiness to leave the government if Johnson becomes prime minster.
Johnson is widely expected to defeat Foreign Secretary Jeremy Hunt in the Tory leadership race, with European Union officials already reportedly preparing to offer him an extension on Brexit in the event that he becomes prime minister. The Conservative Party leadership election was triggered in late May, when Prime Minister Theresa May announced that she would resign as party leader once a successor had been picked. Ten candidates initially put themselves forward for the job, with the final runoff taking place between Johnson and Hunt. The party membership has been voting for the leader by postal ballot, with the result to be announced on Tuesday.
The UK voted to leave the EU in 2016, with the vote resulting in political chaos in the government and the resignation of two prime ministers and dozens of ministers. The UK was initially set to withdraw from the bloc in March, but parliament has yet to endorse a Brexit deal that has the approval of both London and Brussels.
Earlier this month, Philip Hammond warned that a no-deal Brexit would cause a £90 billion "hit" to the Treasury.