Philip Hammond has told the BBC he intends to resign as chancellor if Boris Johnson becomes the UK’s next PM.
He said a no-deal Brexit, something Mr Johnson has left open as an option, was “not something I could ever sign up to”.
He said he intends to quit after Prime Minister’s Questions but before Mrs May steps down.
Mr Hammond said it was important the next PM and his chancellor were “closely aligned” on Brexit policy.
“That is what is likely to happen, and I’m making my plans accordingly”, he said, adding he would wait until the result is announced on Tuesday to “see for sure”.
Mr Johnson has said the UK must leave the EU by the new Brexit deadline of 31 October “do or die, come what may”.
He said: “That is not something I could ever sign up to. It’s very important that a prime minister is able to have a chancellor that is closely aligned with him in terms of policy”.
He added that Jeremy Hunt’s position regarding a no-deal Brexit was “more nuanced”, and he had not demanded a “loyalty pledge” on the exit date from prospective ministers.
Mr Hammond has been a prominent critic of the idea of a no-deal Brexit, recently indicating he may vote to bring down the next PM to stop such a scenario.
Asked on the Andrew Marr programme whether he would vote against the next PM in a vote of no confidence, he said: “I don’t think it will get to that”.
“I am confident that Parliament does have a way of preventing a no-deal exit on October 31 without parliamentary consent”.
Earlier, Justice Secretary David Gauke reiterated his intentions to resign from government should the next prime minister pursue a no-deal Brexit.
Mr Gauke told the Sunday Times : “If the test of loyalty to stay in the cabinet is a commitment to support no-deal on October 31 – which, to be fair to him, Boris has consistently said – then that’s not something I’m prepared to sign up to.”