Theresa May has been warned that Tory rebels have enough support to win tomorrow’s vote on whether Parliament should have a say on final Brexit deal terms. The crunch vote could give MPs the power to delay Brexit.

The Prime Minister has insisted that, when it comes to the final Brexit deal, the MPs’ vote will be a “take it or leave it” offer – arguing nothing can stop withdrawal in March 2019.

But tomorrow’s vote is crucial because, in the event that no satisfactory trade deal is struck by Brexit day in March 2019, it could be used as a weapon to extend the Article 50 negotiations.

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Dominic Grieve, the former Attorney General, urged the Prime Minister to back down and give Parliament a binding vote over the exit terms and timing, rather than suffer an embarrassing defeat.

“I don’t see any possibility of me backing down on this at all. One has to stand up for one’s principles,” Mr Grieve said.

“It could be possible for Parliament to say to the Government: ‘I’m sorry I don’t think you have negotiated a good enough deal’.”

“If our European partners are prepared to continue negotiating, that might also be possible. We just don’t know.”

When asked how many Tory MPs were behind him, he replied: “I think enough, if this comes to a vote, to defeat the Government.”

Further pressure was applied by Nicky Morgan, the former Education Secretary, who confirmed she would back the amendment “with a number of other Conservative MPs”.

“It goes without saying that, in ‘taking back control’, that control should come back to the UK’s sovereign Parliament and that Westminster MPs should have the same final say as our counterparts in the EU,” Ms Morgan said.

Heidi Allen, another Tory MP, also tweeted her support, saying: “Ensuring Parliament has a vote on the final deal will in my opinion, simply allow MPs to do their job.”

Around 20 Conservatives have signalled a readiness to defy the Prime Minister on a binding – or “meaningful” – vote, threatening a first defeat on the EU Withdrawal Bill.

No 10 has hinted at a climbdown, desperate to avoid a damaging defeat on the eve of the EU summit that will give the Government “sufficient progress” to move the Brexit talks onto future trade.

However, pledges of revolt from pro-EU Tories have melted away in the past after heavy arm-twisting from Conservative whips.

Mr Grieve said the Government had “accepted the principle” that the UK cannot leave the EU without MPs giving their approval in a Bill to enact the withdrawal agreement.

Yet, it remained the case that Clause 9 in the EU Withdrawal Bill “allows for the Government to do it all by statutory instrument if it so choses”.

Mr Grieve described that situation as “plainly wrong”, telling Radio 4’s World at One programme: “And that’s why I hope the Government will think again about it.”

MPs fear the promised “meaningful vote” will take place too late to send the Prime Minister back into Brussels talks, unless they also have the power of delay.

Crucially, the promised Bill will be open to amendment, perhaps to later attempts to keep the UK in the EU single market and customs union.

Matthew Pennycook, Shadow Brexit Minister, said: “Tory rebels have talked the talk, now they must walk the walk.