Senior Tory MPs on both sides of the Brexit debate rallied colleagues on Sunday to support the Prime Minister in votes on the EU Withdrawal Bill – or risk letting Jeremy Corbyn into power.
The legislation repeals the law that took Britain into Europe in 1973 and transfers EU laws into British law to provide legal continuity after Brexit day next March 29.
But a series of amendments made by the House of Lords and new proposals from Labour risk shackling the UK to Brussels rule and could even stop Britain leaving if Parliament does not back the final deal.
Tory whips are cautiously confident but not certain they can persuade enough pro-Remain rebels to hold back from helping Labour inflict a damaging defeat on the PM.
Mrs May will speak to a meeting in Parliament of the powerful 1922 committee of backbenchers on Monday.
She is expected to urge them to remember that anxious voters are watching what they do.
“The purpose of the EU Withdrawal Bill is simple: it is putting EU legislation into law to ensure a smooth and orderly transition as we leave,” she is set to tell them.
“But the message we send to the country through our votes this week is important.
“We must be clear that we are united as a party in our determination to deliver on the decision made by the British people.
“They want us to deliver on Brexit and build a brighter future for Britain as we take back control of our money, our laws and our borders.”
A particularly painful few days for the Government last week including leaked comments by Foreign Secretary Boris Johnson suggested he thought Mrs May lacked the “guts” to negotiate a clean break from the EU and that a Brexit “melt down” was looming.
Meanwhile Brexit Secretary David Davis and others were poised to quit until she agreed to put an “expected” end date on how long the UK would stay in EU customs rules pending an alternative being ready – though ministers disagree on how firm a guarantee the date is.
Ahead of the debates planned for Tuesday and Wednesday on the Bill, veteran europhile Tory Ken Clarke urged fellow rebels to hold their nerve.
He claimed defeating the Government would strengthen Mrs May’s ability to face down the hardliners and seek “a softer, sensible Brexit.
“The Cabinet Brexiteers behaved quite disgracefully last week,” he told the BBC’s Sunday Politics.
“What we need to do is to rescue the Prime Minister from this terrible treatment she is getting from key members of her Cabinet.”
But others appeared to be backing down for fear of toppling Mrs May and letting a hardline Brexiteer take over.
Former Home Secretary and Remain campaigner Amber Rudd and ardent pro-Leave former minister Iain Duncan Smith united in an unusual joint appeal to colleagues to stay loyal to the PM.
They said voting to overturn or dilute Lords’ amendments should be a “no brainer” – and Labour would exploit any Government setback for its own purpose.
“It behoves us all to demonstrate discipline and unity of purpose in support of the Prime Minister,” they wrote.
“We cannot allow ourselves to become divided and risk losing the precious chance to go on implementing policies that transform lives.”
Mr Davis, who resumes talks in Brussels on Monday with EU negotiator Michel Barnier, stressed on Sunday this week’s votes were “about respecting the referendum result and respecting the manifesto all Conservative MPs stood on”.
Pro Brexit Housing Minister Dominic Raab said: “People thinking about voting against the Government this week need to think very seriously.”
Mr Raab said the Cabinet should be more disciplined, adding to Sky News: “Everyone needs to get behind the Prime Minister. We need to show as a Government and also as a country that we’re bigger than the sum of our parts.”
Labour Shadow Brexit Secretary Sir Keir Starmer appeared to acknowledge the Government was likely to win the day as he assured MPs wanting to keep close EU links that this week was not their “last chance saloon”.