Brussels has again ruled out making changes to the controversial Irish border “backstop” in Theresa May’s Brexit deal as senior Tories began talks on alternatives.
The European Union's chief negotiator Michel Barnier said the backstop was the "only operational solution" to prevent a hard border on the island of Ireland.
And senior Brussels official Martin Selmayr insisted there were no plans to offer legally binding assurances to help Mrs May get her deal through Parliament. He added that the EU did well to start no deal preparations in December 2017.
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 the Prime Minister, 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.
Ireland’s deputy premier Simon Coveney said alternatives to the border backstop represent “wishful thinking” and that many hours were involved in coming up with a “legally credible and pragmatic” solution.
He added: "He added: "What Ireland is being asked to do by some in Westminster is to essentially do away with an agreed solution between the UK Government and EU negotiators and to replace it with wishful thinking and I think that's a very unreasonable request to ask the Irish Government to be flexible on.
"So if there are alternative arrangements that can work the current protocol, if people take the time to read it, takes account of that and it says very clearly that the backstop can be replaced by alternative arrangements as long as they work."
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 Mr 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, Mr 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."
Mr 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.
Mr Rutte said: "The Withdrawal Agreement remains the only way to ensure an orderly withdrawal of the UK from the European Union.
"We continue to urge the UK Government to clarify its intentions with respect to its next steps."
Mrs 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 Mrs 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, Mrs 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."
Mrs Merkel's visit for talks with Japanese PM Shinzo Abe came soon after a new EU-Japan trade deal came into force and a day after Nissan confirmed it was ditching plans to build its X-Trail SUV in Sunderland.
The company said the decision was largely driven by changing demand for diesel models but added that "uncertainty around the UK's future relationship with the EU is not helping companies like ours to plan for the future".
The chairman of the Brexit Committee, Hilary Benn, voiced scepticism about the prospects for a technological solution to the border issue, telling reporters: "Personally, I don't see how it can work - particularly in the very short amount of time that there is left."
Speaking after the committee's meeting with Mr Selmayr, Mr Benn said the UK Government and EU had already put a great deal of effort into exploring a technological solution, only to conclude that: "It's not going to work."
Meanwhile, Downing Street poured cold water on speculation over an early election, saying that Mrs May was "absolutely not" considering a vote on June 6.
The comments came as Justice Secretary David Gauke became the latest Cabinet minister to suggest that Brexit might have to be delayed beyond the scheduled date of March 29, telling the BBC that the "key" was achieving a "smooth and orderly departure".