Faced with a revolt by up to 40 Tory MPs, the Prime Minister yesterday 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.
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.
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, yesterday 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.