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.
And the prime minister’s spokesman said on Friday that the government wouldn’t extend Article 50, the legal mechanism taking Britain out of the EU on 29 March.
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.
In those circumstances, his amendment means that the Commons will then have a chance to vote on alternative policies – everything from a “managed no-deal” to a further referendum could be on the table.
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.
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…