The MP, who wants a second referendum, said the current date for the UK’s departure from the EU should be removed from domestic law if Mrs May loses next week’s meaningful vote.
He told the BBC: “The first thing we could do is to take the 29th March out of our domestic legislation, because without doing that there’s no point in going to the EU and asking for an extension because we would still be crashing out and that would have to be, I think, a top priority.
“I believe the EU will extend Article 50 for us but I think it’ll only do it in a very limited number of circumstances and we have to be realistic about that as well. So we need to explore what those circumstances might be and have an opinion so that how we can proceed from there.”
The MP for Beaconsfield also spoke at an “emergency convention” on a second EU referendum at Emmanuel Centre in Westminster on Friday.
Grieve, who tabled the amendment that led to a government defeat on Wednesday, told a packed hall that once Prime Minister Theresa May’s deal is defeated in the Commons the only option is a second referendum.
He also said that Conservatives had “blown it” by letting a “revolutionary change take place in the name of tradition”.
Mr Grieve said that if MPs vote against Mrs May’s deal as is predicted next week, then the only option is to go back to the British public and hold a second referendum.
He added: “My friend the Prime Minister has been doing her very best to try to find a way out of this problem that minimises the damage while trying to honour the referendum result.
“The unpleasant truth for her is that it can satisfy no one.
“There is only one way out. When the PM’s deal is defeated, what else can we possibly offer to the British public which has any chance at all, but to go back to ask them to reconsider their decision.
“Parliamentarians do have some duties and one of them is to prevent people from committing national suicide.”
Mr Grieve received a standing ovation from the packed hall when he stepped on stage and again when he finished speaking.
He told the audience that options such as a Norway-style deal or customs union with the EU just “brush problems under the carpet” and the only option was to go back to the public.
But Mr Grieve did not say what questions should be on a second referendum or whether a no-deal Brexit should be on the ballot paper.
Mr Grieve said he had told sixth form students that what it meant to be a conservative was to believe in principals of “continuity” and “pragmatism” and to try to avoid “revolutionary changes”.
He added: “I then explained to them that we had blown it because we allowed a revolutionary change to take place in the name of tradition, and such things can never work. It ends up where you do not want to be.”
Former Green Party leader Caroline Lucas told the audience the referendum of 2016 had highlighted the problems at the heart of British democracy which can no longer be ignored.
The Brighton Pavilion MP said people voted with a “collective howl of rage” but it was also a collective message that the status quo was “intolerable”.
Ms Lucas added: “People were and are angry for many reasons, a general sense that any change is better than the status quo, that they have nothing left to lose. The tragedy of course is they do and likely will.
“We must also be very honest, we know that immigration has been a good thing for Britain.
“The point is that its benefits have not been equally shared and big changes to our communities can be frightening.
“The tragedy is of course is that Brexit would make it harder to address all of those concerns.”
Ms Lucas said a second referendum could “fire the starting gun” on a new conversation about how the country is governed.
She called for a new social contract, better jobs and green investment to address the “grotesque inequalities” and proposed introducing proportional representation to replace the first-past-the-post voting system.
She added: “The lie at the heart of the leave campaign was that this downward spiral could be reversed by leaving the EU.
“The serious democratic deficit in our own country, Brexit has laid bare the extent to which our government structures are derelict.
“A people’s vote should be the starting gun on the race to genuinely democratise the UK.”
Additional reporting for Press Association.