The Prime Minister called the German Chancellor on Tuesday as efforts to design a scheme to avoid creating a “hard border” with new customs checks between Ireland and Northern Ireland reach crunch point.
It is thought to be the last sticking point for agreeing a “divorce deal” with Brussels.
Once agreed, the two sides will focus on finalising an outline vision for future trade relations.
The whole package is likely then to go to an EU summit later this month or in December, and if approved Mrs May will put it to the House of Commons for a vote within days.
Prime Minster Mrs May will attend a working dinner in Belgium on Thursday hosted by Secretary General Jens Stoltenberg.
The event, which comes on the eve of a weekend of commemorations marking the 100th anniversary of the end of the First World War, follows up on discussions at July’s summit of the defence alliance.
But some other EU leaders are likely to be there and it is possible Mrs May could have Brexit-related chats with them on the sidelines of the event.
On Wednesday Mrs May spoke to Donald Tusk, who chairs EU leaders’ summits and will play a crucial role in calling a gathering if enough progress is deemed to have been made in Brussels’ talks with Britain.
British PM Theresa May called the German Chancellor Angela Merkel to help seal a Brexit deal
Theresa May talks with the president of the European Council Donald Tusk
There are claims Mrs May needs agreement with her Cabinet by Monday to trigger a deal-making summit this month and ministers are on standby for a snap summons to Downing Street.
Ministers were invited into the Cabinet Office in Whitehall to view a near-complete draft of the proposed EU withdrawal agreement.
The document, which they saw in strict secrecy and were not allowed to take copies of, is believed to contain the “95 percent” of the exit deal Mrs May says has been agreed with Brussels.
But it is not believed to have included the proposed Irish border solution.
Leaving the building, Environment Secretary Michael Gove told reporters it was “a great document” but did not elaborate.
Increasingly influential Attorney General Geoffrey Cox was tasked at Tuesday’s Cabinet meeting with drawing up a scheme to ensure that any customs union deal the UK enters into with Brussels, to avoid a hard Irish border, will be one that Britain can get out of.
Mr Gove is among Brexiteers demanding that ministers see the full legal advice before approving such a deal.
The Democratic Unionist Party went further saying Mrs May should break with Government convention and make the legal advice public.
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“If the House of Commons is going to have a meaningful vote on a deal, on which this legal advice is very, very important, then I think people are entitled to know what that advice is,” said the party’s chief whip Sir Jeffrey Donaldson.
He futher added to Mrs May’s headaches by warning her she cannot rely on his party’s 10 MPs voting to support her deal – despite the agreement that they prop up her minority administration in crunch votes.
Sir Jeffrey rejected assumptions his party would not do anything to hasten a snap election – which Labour will seek if the deal is rejected – because it does not want to risk Jeremy Corbyn, who has past links with Irish republicans, becoming PM.
Sir Jeffrey told the BBC: “If we think a Brexit deal is not good for the UK, we will say so. Let’s see what the deal is. We’re not afraid of a general election.
Former Conservative PM David Cameron said he “fully supported” Theresa May
“I’m quite happy to go to the people of Northern Ireland on the basis that we voted against a deal because it was not in the interests of Northern Ireland. We want the UK to stay together, and we are not alone in this.”
Former Labour Prime Minister Tony Blair in Portugal repeated his call for a second referendum to give people the chance to avoid Brexit, in either its “pointless” or a “painful” form.
He said a referendum would be best if Parliament rejected Mrs May’s deal, because the Tories would not want an election and a no-deal Brexit was “obviously absurd”.
Former Conservative PM David Cameron was spotted in Whitehall heading to the Treasury to see documents for his memoirs.
He said he “fully supported” what Mrs May was doing and wanted “to let her get on with the job”.
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