Dominic Grieve said he doubts he could remain in the Conservative Party should Boris Johnson win the leadership race.
He told talkRADIO’s Dan Wootten it “may be very difficult for me to remain”.
“I don’t think Boris Johnson is a person who can be trusted, and that comes not from Brexit, but from a long experience of how he goes about his political operation,” he said.
“I think it may be very difficult for me to remain but I don’t wish to leave the Conservative Party of which I’ve been a member for 47 years.”
He added: “I would, I think, wait to see what he has to say and what he does.”
Mr Grieve has been a long-term campaigner for a second Brexit referendum and said a no-deal Brexit would be “uniquely damaging” for Britain.
“It’s going to damage us economically; it’s going to damage our national security and the international standing, indeed that’s already happening even with the prospect of it,” he said.
“My view is that we are not going to get a better deal or different deal by threatening to do something to ourselves which is so disastrous.”
The Member for Beaconsfield has urged the next Prime Minister to reconsider forcing through a no-deal Brexit if he is unable to win a vote in Parliament.
Both candidates have said they want to avoid no deal but need to keep it as an option during EU negotiations.
Mr Grieve said the incoming leader must not relegate the House to the sidelines “in complete breach of every constitutional principle under which our parliamentary democracy operates”.