REBEL Conservative MPs are warning Theresa May that they could combine with Labour and the SNP to amend the Brexit Bill unless the UK Parliament is given a say on the "endgame" negotiations with Brussels.

The warning has come as Alex Salmond, the former First Minister, called on other parties to “build a cross-party progressive alliance, that will call a halt to this Mad Hatter's Westminster tea party”.

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"But let this be a clear warning to the UK Government: the SNP will fight tooth and nail to blunt the axe of a hard Brexit falling again this week.”

And he again warned of the prospect of a second Scottish independence referendum if the Prime Minister and her colleagues did not heed the SNP’s proposals for a differentiated Scottish deal.

"The UK Government has set about severing links with one Union already and if it continues to act and push ahead with its hard Brexit, then it can be sure that it will be testing the strength of another.

“Scotland,” added the Gordon MP, “has no intention of allowing our thousand-year history as a European nation to be severed by a bunch of Westminster, born-again Brexiteering Tories, who have taken leave of their senses.”

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MPs are preparing for a three-day debate in the Commons, which will cover more than 100 pages of amendments, although the vast majority are unlikely to be voted on because of the restricted nature of the EU Notification of Withdrawal Bill, which is merely a technical device to allow the UK Government to trigger Article 50. Last week, MPs voted by a healthy majority to back the bill in principle; starting today they will begin to scrutinise the issues in more detail.

Pro-Remain Tories fear the Prime Minister’s warning she would prefer "no deal to a bad deal" could see Britain "falling off a cliff" if negotiations broke down.

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Backbencher Neil Carmichael expressed concern that, as it stood, MPs would be unable to force Mrs May back to the negotiating table with potentially disastrous consequences for the economy.

Writing in The Mail on Sunday, he warned that he was not the only Tory MP to feel strongly about the issue.

"I hope to persuade the Government to meet our concerns this week before the Brexit Bill is finalised," he said.

The prospect of a Government defeat has infuriated pro-Brexit Conservatives, who have accused Tory rebels of trying to derail the Bill.

Steve Baker, a leading Tory Leave campaigner, suggested there were as many as 27 Conservative MPs who could vote with Labour and the SNP.

"This is a time to unite behind a democratic result not plot to repudiate it. Any vote to amend this simple Bill is a vote against implementation of the referendum result," he said.

But that figure was dismissed as being far too high by the pro-Remain camp. Yet with a Government working majority of just 16 in the Commons, the voting arithmetic may be tight even if Northern Ireland’s eight Democratic Unionist MPs side with the PM.

Former ministers Anna Soubry and Dominic Grieve were among those reported to be considering voting for an amendment alongside the veteran pro-European Ken Clarke who was the lone Tory to rebel at second reading.

In his article, Mr Carmichael said that in his view, Parliament had to have a final say when they reached "the endgame" in the negotiations.

"We could be faced with the prospect of leaving the EU by 'falling off a cliff' - as some have described leaving with no deal - with potentially disastrous economic consequences," he wrote.

"But MPs would have no chance to say the Government should step back from the brink and return to the negotiating table.

"To argue that MPs can have a say if we achieve a deal, with all the safeguards implied by it, but no say if we walk away with none of these, defies logic.

"If that occurred, it would be even more important for MPs to be able to vote for or against it."

Ms Soubry said she shared the concerns as to what would happen if there was no deal and dismissed critics who claimed they were trying to wreck Brexit as "mad conspiracy theorists".

"If there is no deal then the Government will determine what happens next. I think it should it come into Parliament. I don't know why people are so fearful of that," she told BBC1's The Andrew Marr Show .

For Labour, Emily Thornberry said her party would not seek to block Article 50 even if it did not get its amendments accepted.

"We have said that we will not frustrate Brexit. We have got our instructions from the British people. We are democrats and the public have voted to leave the European Union," she told The Andrew Marr Show.

"There are going to be negotiations happening in the next week. There are many ways in which the Government may be able to react to this that will be positive.

"On one of the amendments we have put down they may say, 'We're not going to support this amendment but during a speech we can give an assurance, we can speak in back channels, we can say you will get this.'”

The Shadow Foreign Secretary added: "There will need to be back channels, private conversations. There are many conversations going on now. We are speaking to Government, we are speaking to Tory backbenchers and we are trying to get a compromise that will work."

Ms Thornberry refused to be drawn on whether Diane Abbott, the Shadow Home Secretary - who infuriated many Labour MPs when she missed last week's vote because she said she had migraine - would face the sack if she failed to support the Bill in Wednesday's final Third Reading vote.

"It is a fast-moving picture. Let's see what happens," she said. "The whip will be decided next week. Let's see what happens in relation to the amendments. We have a Shadow Cabinet meeting next week. It will be for the Chief Whip and the leader to decide what the whip is."