Theresa May is facing a potentially humiliating backbench rebellion as part of a bid to force her to reveal her plans for Brexit.

The vote will be seen as a key test of the new Prime Minister’s authority.

Former government minister Anna Soubry is among those who have indicated that they are prepared to support a Labour Party motion to make Mrs May’s administration set out its proposals for the negotiations to leave the European Union.

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Ms Soubry, who campaigned to stay in the EU, said that as many as 40 Conservative MPs could join her to vote against their own government, on what would be a significant blow for the Tory leader.

Labour sources also called on SNP MPs to support the motion.

Keir Starmer, the shadow Brexit Secretary, said that the move was a “real opportunity to finally get clarity on the Government’s plan for Brexit”.

But the UK Government said that it was the way to ensure the "worst" possible outcome for the UK.

Mrs May has said that her government will not set out its plans, for fear of giving its away its hand to other European nations.

The UK is expected to face tough exit talks with the other 27 EU member states, some of whom are keen to ensure that Mrs May does not achieve too good a 'divorce' settlement.

Critics say that Mrs May's strategy has left the public, and opposition parties, in the dark over the government's ultimate aim.

The Conservative party is split over the benefits of a ‘hard’ Brexit, in which immigration controls are prioritised over access to the EU's Single Market, and its opposite, a so-called ‘soft’ Brexit.

They complain that although the referendum clearly told ministers that the UK people wanted to leave the EU it offered no clarity on what a new relationship with the rest of Europe should look like.

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Ms Soubry said that there was nothing in the Labour motion "I don't approve of and could not support”.

She urged the Prime Minister to head off the rebellion by putting down her own “sensible amendment”.

"These things are incredibly important," she said. "This actually transcends party politics and tribalism.”

Mr Starmer said: “Labour accept and respect the referendum and we will not frustrate the process of leaving the EU. But Parliament and the public need to know the basic terms the Government is seeking to achieve from Brexit. This issue is too important to be left mired in uncertainty any longer.

“That is why Labour have called this debate. Our motion is simple but would deliver real accountability and grip in the Brexit process. I hope MPs on all sides of the House will join Labour in supporting it.”

The motion recognises that the UK will leave the EU and that no sensitive material should be disclosed before negotiations start.

But it calls for a ‘basic plan’ for leaving the EU to be put before Parliament.

The Prime Minister’s official spokesman rejected that call, saying: “We are not going to set out the terms of our negotiation before we need to do so.

“Where we can set out positions we will but we have been clear we won’t be revealing our cards before we have to.”

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He added: “We have been very clear that setting out the terms of our negotiation before we need to do so is the best possible way to get the worst possible deal.”

Asked if ministers were changing their travel plans to get back for Wednesday afternoon’s debate and vote.