The British government said they will published the "final and full" advice on Wednesday.
On the first day of debate, before the main vote on December 11, her government was found in contempt of parliament and then a group of her own Conservative Party lawmakers won a challenge to hand more power to the House of Commons if her deal is voted down.
Geoffrey Cox, her attorney general, also backed the withdrawal accord in remarks to MPs on Monday.
May planned open the debate by arguing that members of Parliament must back the agreement to deliver on the voters' decision to leave the European Union and "create a new role for our country in the world".
Commons leader Andrea Leadsom confirmed the u-turn over the legal advice after MPs decided her ministers were in "contempt" of Parliament. The petitioners in the case - six Scottish MPs, MEPs and MSPs, along with Devereux Chambers' Jolyon Maugham QC - argue the case will help clarify the realistic options for MPs who will vote on Theresa May's "Chequers deal" a week today.
As he presented his motion to the House, Mr Grieve said: "The House will recall that back in last June issues arose about how the House should proceed in the event of the Government's motion being rejected".
In an opinion issued before a judgement by the European Court of Justice, Advocate General Manuel Campos Sanchez-Bordona rejected the contention that Article 50 can only be revoked following a unanimous decision of the European Council, made up of the EU's member states.
Defeat would leave the United Kingdom facing a chaotic "no-deal" Brexit and could topple the prime minister, her government, or both.
Ms Haddon said the contempt motion was a "show of force" which could foreshadow both the final vote on the deal and the various amendments lawmakers are trying to attach to it.
The advice looked at all the potential legal implications of the current Brexit deal.
They hope this will allow parliament to express its support for alternative approaches - and prevent the government either hurtling towards a no-deal Brexit without the backing of MPs, or imposing a plan B of its own devising.
Meanwhile, Bank of England Governor Mark Carney hit back at "unfair" criticism, after pro-Brexit MPs accused him of scaremongering.
"More and more MPs are concluding that the Government's proposed deal is not what was promised two years ago, it's a much worse deal than the one we've already got in the European Union and, if approved would mean Brexit goes on forever because it leaves all the big questions unanswered".
The full legal advice given to ministers is understood to run to several thousand pages. Lawmakers are due to hold five days of discussion before voting December 11 on whether to accept or reject the agreement, which lays out the terms of Britain's departure from the bloc on March 29 and sets the framework for future relations with the EU.
Mrs May said Britain will leave regardless of any future decision by the EU's top court and that the choice is between her deal or no deal.