‍British Prime Minister Theresa May on Monday postponed a parliamentary vote on her Brexit deal to avoid a crushing defeat, saying she would return to Brussels for further talks – but the EU warned it would not reopen the agreement.

In an emergency statement to the House of Commons, May conceded the agreement she struck with the European Union last month faced defeat by a "significant margin" of MPs on Tuesday.

She vowed "no doubt this deal is the right one" but would seek "further assurances" over a controversial backstop clause relating to Northern Ireland.

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May's crunch Brexit vote: Win, lose or delay?

Theresa May to postpone UK Parliament vote on Brexit deal: reports

May said the date for a new vote depended on fresh talks with EU leaders, set to begin Tuesday in meetings with Dutch Prime Minister Mark Rutte in The Hague and then German Chancellor Angela Merkel in Berlin.

In response, EU President Donald Tusk called a special summit of the other 27 leaders to discuss Brexit on Thursday, at the start of a two-day Brussels meeting that May is due to attend.

But he warned: "We will not renegotiate the deal, including the backstop, but we are ready to discuss how to facilitate UK ratification."

The delay in signing off the deal, just months before Britain ends its four-decade membership of the bloc with Brexit on Mar 29, sent the pound plunging to an 18-month low.

Both May and Tusk also said they would look at stepping up preparations for the potentially catastrophic possibility that Britain leaves the EU without any new legal arrangements in place.

"For as long as we fail to agree a deal, the risk of an accidental no deal increases," May told MPs.