Conservative rebel Dominic Grieve has urged the prime minister to delay Brexit if her EU withdrawal deal is rejected by MPs next week.
Mr Grieve, who backs calls for another referendum, said she could remove the 29 March date from UK legislation and ask the EU for more time.
Cabinet ministers who oppose leaving the EU without a deal had a “duty to resign” if she refused to do so.
Ministers warn the UK faces Brexit “paralysis” if the deal is rejected.
Mr Grieve, who tabled the amendment that led to a government defeat on Wednesday, has been at the forefront of cross-party efforts to ensure MPs have a say in what happens if Mrs May’s deal is voted down.
MPs are widely expected to reject the withdrawal deal negotiated between the EU and UK in a key Commons vote on Tuesday, with more than 100 Conservative MPs among those opposing it.
Earlier Foreign Secretary Jeremy Hunt warned that the possibility of Brexit not happening at all had increased, with Parliament showing this week it would not accept leaving without a deal.
This he said would be an “incredibly damaging breach of trust and it would also be very bad for Britain’s reputation abroad”.
But former attorney general Mr Grieve, who earlier addressed a rally of people calling for another referendum, said he hoped that cabinet ministers known to be opposed to a no-deal Brexit would be “frank” with the PM, if – as expected – she loses next week’s vote.
“I think the options are very limited and the reality in my view is that the only way out of this difficulty is go back to the public and ask for their opinion,” he said.
He said, if MPs reject the deal, the government should act immediately to strike the 29 March Brexit date from UK legislation before going to the EU to ask for an extension of the Article 50 process.
“I believe the EU will extend Article 50 for us but I think they would only do it in a very limited number of circumstances … and we need to explore what those circumstances may be.”
If the prime minister would not agree to do so, he added: “If a cabinet minister feels the government is doing something they can’t accept, it is probably their duty to resign”.
“My feeling is that this will have its own dynamic and whilst I want to keep the government stable … I hope the prime minister will listen carefully to what members of Parliament and members of her own government are saying to her.”
He said it was the duty of MPs to “stop people committing national suicide” by going ahead with a no-deal Brexit.