Brexit Secretary David Davis is expected to face calls in the Commons to reveal how detailed a plan Theresa May will set out before starting negotiations with the European Union.

Faced with a revolt by up to 40 Tory MPs, the Prime Minister on Tuesday bowed to pressure and backed a Labour motion which says she should publish a plan before triggering the formal process of leaving.

In return, most of the rebels and Labour will back a compromise Government amendment to support Mrs May's pledge to invoke Article 50 to start Brexit by April.

Read more: 36% of SNP and Labour supporters backed Brexit, finds survey

Both sides will claim victory in the parliamentary battle, with Labour hailing Mrs May's move as a "significant 11th-hour concession".

But a Number 10 source indicated Mrs May could be hoping to expose die-hard Remain supporters in a vote, saying: "Crucially, from our perspective, it's making sure that Parliament are very clear they are not going to use this as a delaying method.

"So it's now down to MPs to signal that they also want to get on with Brexit by supporting our position."

Downing Street stressed it would not affect the Government's Supreme Court battle to overturn a ruling that it needs Parliament's approval before triggering Article 50, because the vote is on a symbolic motion rather than legislation.

The concession also left Mrs May some wriggle room, because the Labour motion allows her to keep details of the strategy secret if revealing them would damage the UK's position in the negotiations.

Amid speculation surrounding the level of detail Mrs May will set out, Labour urged her to publish the plan by the end of January, in a possible attempt to leave time to further force her hand.

Read more: 36% of SNP and Labour supporters backed Brexit, finds survey

Remain-backing Tories including Anna Soubry called for a White Paper setting out the different Brexit options for MPs to scrutinise.

But Tory former minister Sir Oliver Letwin, who chaired the Government's Brexit unit immediately after the referendum, said he did not expect Mrs May to set out any more detail than what is already known.

He said it was already clear from Government commitments to striking trade deals around the world and having total control over immigration that Britain would leave the EU customs union and single market.

Sir Oliver told BBC Two's Newsnight: "I don't know how long it (the plan) will be, but I'm sure Whitehall will create something mellifluous and mysterious.

"But I very much doubt, and I hope certainly, that it won't say anything very material that hasn't been said already."

Mr Davis, who will reply to the debate for the Government, is also likely to face questions about the apparently accelerating timetable for negotiations, after the EU's lead Brexit official warned the UK will have to reach a deal within 18 months.

European Commissioner Michel Barnier urged the UK to "keep calm and negotiate" as he suggested October 2018 will be the deadline for agreement so any deal has time to be ratified by the European Parliament and national leaders sitting in the European Council.

The question over whether Britain can really "have its cake and eat it" is also likely to arise after Mr Barnier said the UK will not be allowed to "cherry-pick" which EU rights and obligations it wishes to keep, suggesting it cannot stay in the single market if it does not accept free movement.

The suggestion of transitional trading arrangements was first raised by Mr Davis last week and he is likely to face calls for more clarity.

Shadow Brexit secretary Sir Keir Starmer will lead the debate for Labour and is expected to call on the PM to set out her "basic plan".

Speaking to the Scottish Parliamentary Journalists' Association, he said: "I've never asked for a running commentary and I do appreciate when you go into negotiations you don't want to give away your detailed negotiating position.

"But there are headline issues upon which we need to have answers. Are you aiming to be in or out of the customs union? What is your position on the single market, what about transitional arrangements and what about the residual rights of EU nationals? Just basic headline issues we need to have the answer to."

Mrs May has so far played her cards close to her chest, on Tuesday committing only to a "red, white and blue Brexit" in an attempt to stamp out speculation about whether the Government is aiming for a "hard" or "soft" Brexit, outside or inside the single market.

A further amendment backed by Scottish National Party and Plaid Cymru MPs calls for "a formal role for the devolved administrations including their agreement before Article 50 is triggered", while the Liberal Democrats have tabled a demand for a second referendum on the outcome of negotiations.

SNP Europe spokesman Stephen Gethins attacked Labour for backing the "hard-right Tory" Government amendment.

"It has been 167 days - almost six months - of distraction and deflection from the UK Government, and just 113 days until 31 March deadline," Mr Gethins said.

"Today's debate is welcome in holding the Government to account over its inaction.

"However, Labour have a decision to make - they can either back our amendment to the motion that respects the vote across all four parts of the UK and that will give devolved administrations a formal role in the negotiations and seek agreement before triggering Article 50.

Read more: 36% of SNP and Labour supporters backed Brexit, finds survey

"Or they risk backing a Tory amendment that will see the UK put through a hard-right Tory plan to take us out of the EU that will damage jobs, livelihoods, businesses and the economy."

Green Party co-leader Caroline Lucas said she would vote against the Government amendment to back Mrs May's timetable for triggering Article 50.

She said: "Yesterday the Prime Minister's plans developed in a laughable fashion - they went from 'having our cake and eating it' to a 'red, white and blue Brexit'.

"For MPs to be asked to vote in favour of triggering Article 50 in March with such extreme reluctance from the Government to give any details of the plans beforehand would, in my view, be reckless - and I won't be gambling with this country's future by falling into this Tory trap."

The Lib Dems also said they would vote against the Government unless it committed to publishing a White or Green Paper setting out its plans, including details on whether it aimed to keep the UK in the single market.

Party leader Tim Farron said: "We cannot support a parliamentary stitch-up that would deny the people a vote on the final deal.

"An amended motion would fail to include any meaningful commitment from the Conservative Brexit Government to produce the equivalent of a White or Green Paper setting out its position on such fundamental questions as to whether it wants Britain to remain in the single market.

"I call on the Labour Party to remember it calls itself the Official Opposition.

"It should not cave in to Conservative attempts to deny the public a final say on the most important question facing the country in a generation."

Labour MPs David Lammy and Ben Bradshaw said they would vote against triggering Article 50 by April.

Mr Lammy will say in the debate that he cannot back the Government amendment until he has assurances that a detailed plan will be set out beforehand.

He will say: "Does this mean that the Government are just going to publish a document saying 'We will seek the best possible Brexit, and aim for the best possible access to the single market' and then say 'There you go: there's our plan'?

"Because we have heard that before and that is not good enough."

Mr Bradshaw tweeted: "I will not vote today to invoke Article 50 by March when we still have no idea what sort of #Brexit the Government will pursue."

Downing Street claimed Mrs May's "plan all along" was to set out the Government's negotiating position before beginning Brexit, pointing to the PM's Tory conference speech in October in which she "made very clear that we will set out clarity where we can".

A Number 10 spokesman went on: "We've done that, in terms of identifying when we're going to trigger Article 50; the Prime Minister was also clear that once we have more clarity on issues we will provide that."

Asked if this meant Mrs May currently did not have clarity on some issues, the spokesman replied: "We do have clarity; what I'm saying is where we have been able to provide clarity we will and we will continue to do so."

The spokesman also said he would not speculate what form the plan would take when asked if it would be a White Paper.