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Rebel Tory MPs delivered an ultimatum to Theresa May today by declaring the Government’s EU Withdrawal Bill “not fit to be passed”.

They raised the stakes for the Prime Minister by making clear her “rule by decree” legislation risks being shot down in the Commons without major changes.

Senior Conservatives say they are surprised Mrs May has not yet contacted them to offer reassurances or concessions after it became clear her Commons majority could be overwhelmed. A key demand for a binding vote in Parliament on Britain’s withdrawal terms has emerged as one of the biggest dangers for the Prime Minister.

Writing in today’s Standard, former attorney general Dominic Grieve urged Mrs May to accept that the Bill is flawed, including an “excessive” use of so-called Henry VIII powers that allow ministers to change laws by diktat. “It ought to be possible to achieve a sensible consensus on this,” he said. “Without it the Bill is not fit to be passed.”

The former minister argued that uniting Parliament is vital if Mrs May wants to succeed in Brexit negotiations. The EU Withdrawal Bill returns to the House of Commons for debate shortly. But the Government has delayed the crucial committee stage after being deluged with 300 amendments, many of them commanding cross-party support.

Although the Bill does not cover key questions such as whether the UK will have access to the European single market, those issues would be centre stage if the Government is forced to allow a vote on the final deal.

Responding to Mr Grieve’s article, Lib Dem leader Sir Vince Cable said: “This is a devastating blow to the Government’s plans for a divisive hard Brexit. Dominic Grieve is absolutely right.” Shadow chancellor John McDonnell said MPs would unite to force a “meaningful vote” on the terms of exit.

MPs say the Bill could allow ministers to dump by stealth existing EU laws that guarantee equality for sex and gender. “I am satisfied that the Government has no such intention,” Mr Grieve said. “But accepting amendments to prevent their alteration save by primary legislation would be both conciliatory and sensible.”

Mr Grieve emphasised that the rebels were not trying to halt Brexit, but protect liberties. He said: “Some in favour of Brexit are so fixated on leaving the EU, they keep arguing that any attempt to change it is some form of sabotage. It is not.”

The Withdrawal Bill cleared its first Commons test last month in a historic vote in which the rebels stood back to give time for Mrs May to offer concessions. It passed with a majority of 36 thanks to a Labour split.

Tory rebel heavyweights, including Mr Grieve, former chancellor Kenneth Clarke, ex-minister Bob Neill and four committee chairs, including Treasury committee chief Nicky Morgan, have said they will not back amendments put down by Jeremy Corbyn but that Labour must unite behind their proposals.

Key demands include for Parliament to have a meaningful vote on the final withdrawal terms rather than the “deal or no deal” vote offered by No 10.

Mr Clarke said a no-deal situation would be “catastrophic” for the UK economy in the short term. In a dig at Foreign Secretary Boris Johnson, he said: “We mustn’t be seduced by this bizarre stuff of the lion roaring, and of President Trump, and the Japanese, and protectionist countries around the world, all being prepared to open their markets to us, no rules we have got to comply with, and no obligations... this is compete fantasy. It’s la-la land.”

However, former Welsh Secretary John Redwood said Britain would be “fine” without a deal. Mrs May was holding emergency talks in Brussels today in an attempt to get “deadlocked” Brexit negotiations back on track. She was dining with EU chief negotiator Michel Barnier and Commission president Jean-Claude Juncker in a meeting that came as a surprise to observers.

The Commission said the dinner would not take the place of formal talks. The last time the two sides had dinner together, Mr Juncker reportedly called Mrs May “deluded” over Brexit. No 10 said: “We are looking forward to a very constructive dinner.”

Mrs May will also speak to Irish premier Leo Varadkar and French president Emmanuel Macron today. She spoke on the phone with German chancellor Angela Merkel yesterday. No 10 said the pair agreed “on the importance of continued constructive progress in the UK’s exit negotiations”