Voting will close later in the Conservative leadership contest, with the UK’s next prime minister set to be announced on Tuesday.
Party members will have until 17:00 BST to return their ballots for either Jeremy Hunt or Boris Johnson, regarded as the frontrunner in the contest.
The winner and successor to Theresa May is due to take office on Wednesday.
On Sunday, Philip Hammond told the BBC he intends to resign as chancellor if Mr Johnson becomes prime minister.
Mr Hammond told the BBC’s Andrew Marr Show he could not “sign up” to Mr Johnson’s Brexit strategy, which requires leaving the EU by 31 October, with or without a deal.
His announcement followed Justice Secretary David Gauke reiterating in the Sunday Times that he would also resign this week for the same reason.