Brussels has stepped up its opposition to attempts to reopen Theresa May's Brexit deal as senior Tories begin talks on alternatives to the Irish backstop.
The European Union's chief negotiator Michel Barnier says the backstop is the "only operational solution" to prevent a hard border on the island of Ireland.
And senior Brussels official Martin Selmayr insists there are no plans to offer legally binding assurances to help May get her deal through parliament.
The EU's refusal to give ground came as senior Tories from both wings of the party met in Whitehall for talks aimed at finding a solution to the Irish border issue following last week's Commons vote calling for "alternative arrangements" to replace the backstop.
In another headache for May, former first minister of Northern Ireland Lord Trimble threatened to take the government to court over the backstop, arguing it breaches the terms of the Good Friday Agreement.
The alternative arrangements working group, made up of senior Leave and Remain-leaning Tories, held its first talks in Whitehall with Brexit Secretary Stephen Barclay.
After more than two hours of discussions, former Tory leader Iain Duncan Smith, a leading Brexiteer, said "all sides of this debate" were "coming together to find a solution", while ex-education secretary Nicky Morgan said the talks were "very constructive".
Meanwhile the cross-party House of Commons Brexit Committee was in Brussels for talks with senior figures including Selmayr, the European Commission general secretary viewed as Jean-Claude Juncker's right-hand man.
In response to reports that he had told the Brexit Committee that the EU would be ready to consider legally binding assurances, Selmayr tweeted: "On the EU side, nobody is considering this.
"Asked whether any assurance would help to get the Withdrawal Agreement through the Commons, the answers of MPs were ... inconclusive.
"The meeting confirmed that the EU did well to start its no deal preparations in December 2017."
Barnier held talks with Dutch Prime Minister Mark Rutte and said there was "full agreement that (the) Withdrawal Agreement cannot be reopened".
He said the EU was "ready to work on alternative solutions during transition", restating Brussels' position that the backstop had to remain in place unless and until a replacement could be agreed.
May will visit Northern Ireland on Tuesday for a speech in which she is expected to confirm her government's "absolute commitment" to avoiding a hard border with the Republic after Brexit.
Downing Street has indicated potential solutions could revolve around a time limit or unilateral break clause on the backstop or new technologies to make it unnecessary.
But May's official spokesman declined to say whether Home Secretary Sajid Javid was right to suggest that the UK Border Force had identified "existing technologies" to do the job.
German Chancellor Angela Merkel said the EU was ready to listen to proposals to solve the border "riddle" but needs to hear from Britain how it thinks it can be done.
Speaking during a trip to Japan, Merkel said: "To solve this riddle, you have to be creative and you have to listen to one another.
"We can have those conversations, so we can use the remaining time to perhaps remove the obstacles that have so far stood in the way and find an agreement if everyone is willing.
"But we must hear from Great Britain how they want to do it."
