MPs will vote on the deal and any changes the PM secures tomorrow
The government had claimed that it has secured “legally-binding changes” to the Northern Irish backstop.
The de facto deputy PM David Liddington told MPs that Mrs May had secured meaningful changes to the deal.
Mr Lidington said: “Tonight we will be laying two new documents in the House; a joint legally-biding instrument on the Withdrawal Agreement and protocol on Northern Ireland and a joint statement to supplement the political declaration.”
But Labour’s Shadow Brexit Secretary Keir Starmer warned: “People will be disappointed when they look at the detail.”
The announcement came after Theresa May made a desperate 11th hour dash to Strasbourg to beg EU chiefs to help save her Brexit deal.
Yet Downing Street remains on high alert over fears the Brexit deal – even with a few tweaks – is facing a second catastrophic defeat.
Mr Lidington revealed that the UK and EU will agree a “joint interpretative instrument” to the withdrawal agreement
It reportedly emphasises statements by both sides that the they will use “best endeavours” to end the backstop.
It is also understood to give a key role to an arbitration panel role – who will hear appeals if the UK feels the EU is trying to keep it in the Northern Irish backstop against its will.
Additional lines are also set to be added to the political declaration – based on letters exchanged between May and Juncker on the backstop.
A third plank of a unilateral declaration by the UK, that Brussels and Dublin would have to back or at least not contradict was still being negotiated last night.
Mr Lidington said the Attorney General Geoffrey Cox would be publishing advice ahead of Tuesday’s debate, but needed time to consider the new changes.
“The Attorney General will publish his legal opinion,” he said. “That will be available in good time before the debate.
Mr Cox who was understood to be looking for a reason to reverse his view that the UK could be trapped in the backstop.
The Prime Minister rushed to Strasbourg to try to seal an agreement before the make or break vote on her plan on Tuesday.
Cabinet ministers were being briefed on the developments and the DUP were in discussions with Chief Whip Julian Smith.
But a heavy loss was still expected.
That blow in Parliament would then spark votes to block no deal and a delay to Brexit later this week.
Brussels insiders claimed the PM had been ready to compromise with EU negotiators at the weekend.
But one told the Mirror: “The political situation in London was such that May could not convince her whole Government.”
The source claimed an offer of new legal assurances on the temporary nature of the Irish border backstop was the UK’s “last chance”.
The EU has already rejected proposals to give the UK the right to unilaterally exit the hated no deal insurance policy.
Irish PM Leo Varadkar held an emergency Brexit meeting of the Irish cabinet to discuss developments.
Labour leader Jeremy Corbyn said MPs would not put up with more delay.
“Time and time again this Prime Minister has failed to negotiate, refused to compromise, and delayed and delayed,” he said.
“After three months the Prime Minister has not achieved one single change to her deal; she is simply running down the clock.”
Even if Mrs May manages to strike a last minute compromise on a time limit, there is no guarantee mutinous Tory Brexiteers will back it.
Senior ministers believe her beleaguered plan, which was defeated by 230 votes in January, could be lost by more than 100 this time.
One said: “We’re a long way up shit creek. Never mind a paddle, we don’t have a boat.”