The EU's chief Brexit negotiator said Tuesday that the length of any delay to the divorce that the bloc may grant Britain will depend on what plan Prime Minister Theresa May brings to a crunch summit.

The British leader is making a last-gasp diplomatic dash to Paris and Berlin on the eve of an emergency summit in Brussels, where she hopes the other 27 EU chiefs will agree to postpone Brexit to June 30.

She needs more time to try to strike a compromise with the opposition that will let Britain's bitterly divided parliament back an orderly divorce plan at the fourth attempt. Without such a delay, the United Kingdom is scheduled to crash out with no deal on Friday.

But the EU has already granted one delay -- the original deadline was March 29 -- and negotiator Michel Barnier said May must explain what another postponement would achieve.

"The length of the extension must be linked to the purpose -- what it's for -- and that depends on what Mrs May will say to European leaders tomorrow," Barnier told reporters after a meeting of EU Europe ministers to discuss the Brexit crisis.

"She is coming back to indicate the way forward, that is what is expected by European leaders... It is for her to bring the roadmap." Barnier insisted it would not be the EU's fault if a chaotic "no deal" departure -- predicted to have a calamitous impact -- came to pass.

"No deal will never be the EU's decision, it will always be the responsibility of the United Kingdom to tell us what it wants," Barnier said.

May's talks with the main opposition Labour Party have made little tangible progress, making an imminent breakthrough seem unlikely, but Barnier said he was still hopeful the process could bear fruit.

"The new element in this serious and complex situation is the cross-party talks which have finally been started by Theresa May with Labour," Barnier said.

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