He believes that if the PMs deal on Brexit is not backed by Parliament then the United Kingdom could remain in the EU. As the protectors of our democracy, we simply can not allow this to happen. "If May goes down to defeat and she does not resign and call an election, this is the moment we have to act".
The prime minister has already pushed the crucial vote on ratifying the Brexit package back by a month in a bid to win backing, and it looks like those efforts have failed. But even if she suffered a loss of closer to 200, which many Tories fear could be the case, Conservative MPs and ministers still expect her to stagger on and seek to bring an improved offer back to the Commons for a further vote within weeks.
He said if it is defeated, Britain should continue to press the European Union for a deal that "respects the referendum but if Brussels' "intransigence" persists "we must be willing to leave the European Union at the end of March on World Trade Organisation terms".
Mr Grieve is among a group of MPs calling for a second referendum. Angela Smith, Labour MP for Penistone and Stocksbridge, said: "The time for prevarication is over".
The vote had been scheduled to take place in December but was called off at the last minute by the prime minister, who was facing nearly certain defeat.
But former Brexit minister Dominic Raab said Britain should be prepared to leave with no arrangement in place. He said: "Proposing a no-confidence motion is the first step for either scenario, and we need to get on with it".
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Sir John reiterated his support for a second referendum on Brexit, insisting it would be "morally reprehensible" if the government allowed the country to crash out of the European Union without a deal.
Roy Hattersley, a former deputy Labour leader, came out in favor of another referendum on Saturday.
Meanwhile, pro-Remain cabinet ministers are preparing to push for a softer Brexit this week.
An amendment passed by Parliament means that the Government must come back with their Plan B three days following the vote. This could be one option put forward in a series of indicative votes to test the views of MPs on alternatives if her plan is thrown out.
Mr Hunt said: "It's now looking much less likely that Parliament would allow a no-deal outcome anyway".
But when asked whether he would support revoking Article 50, he said: "Absolutely, I would do what's right".
Meanwhile, Work and Pensions Secretary Amber Rudd said she was "committed" to ensuring that the United Kingdom did not leave without a deal.