EU chief Brexit negotiator Michel Barnier (centre) answers journalists' questions as he arrives in Brussels. AFP

European Union members agreed Monday to postpone Brexit for up to three months, stepping in with their decision less than 90 hours before Britain was due to crash out with no divorce deal.

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The next deadline for departure will be January 31 next year -- although the other 27 capitals would allow an earlier date if London ratifies a withdrawal agreement before then.

Britain's Prime Minister Boris Johnson waves on the steps of Downing street in central London on Monday. AFP

"The EU27 has agreed that it will accept the UK's request for a Brexit flextension until 31 January 2020," the president of the European Council representing member states, Donald Tusk, tweeted.

"The decision is expected to be formalised through a written procedure," he said, after ambassadors met in Brussels to approve the extension.

Leaving the envoys, EU Brexit negotiator Michel Barnier told reporters that "it was a very short and efficient and constructive meeting and I am happy the decision has been taken."

He declined to provide details of the talks.

Donald Tusk said on Twitter "the EU 27 has agreed that it will accept the UK's request for a Brexit flextension (...). The decision is expected to be formalized through a written procedure."

Tusk's announcement came as European Union diplomats met in Brussels to sign off on the new delay to Britain's departure from the bloc, which had been due in just three days on Oct. 31.

It's the second time the Brexit deadline has been changed since the 2016 referendum on Britain's departure from the EU.

Opposition Labour party leader Jeremy Corbyn leaves his residence in north London on Monday. AFP

The French minister for European Affairs says she does not know when Brexit will happen, and insists Britain can still backpedal on its decision to leave the bloc.

Speaking on France Info radio, Amelie de Montchalin said it's still possible for Britain to revoke Article 50, meaning the UK would remain in the EU and cancel Brexit.

Montchalin said "the prime minister can pick up his phone and call Brussels to say: "I stop everything."

Montchalin added, however, that she believes it's important to respect the "British people's sovereign desire" expressed in the 2016 referendum on leaving the EU.

She spoke as ambassadors for the EU27 meet in Brussels on Monday to decide on a new Brexit delay. She said ambassadors will discuss the possibility of granting another extension to Jan. 31, 2020.

Demonstrators take part in a rally in London. File photo

France was initially reluctant to extend the Brexit deadline beyond its scheduled date of Oct. 31, but Montchalin said the perspective of a new general election in Britain justifies a new delay.

European Union diplomats are meeting to decide whether to delay Britain's departure from the bloc, due in just three days.

British politicians, meanwhile, are wrangling over what to do with the extra time.

Ambassadors from the 27 other EU countries are meeting Monday to discuss Britain's request for a three-month postponement to the Oct. 31 Brexit deadline. The bloc has agreed in principle, but has not fixed the extension's length.

In London, British politicians are set to vote on whether to hold an early election to try to break the country's deadlock over Brexit. Prime Minister Boris Johnson wants a Dec. 12 election, but looks unlikely to get the required support from two-thirds of lawmakers.

Two opposition parties plan to push for a Dec. 9 election if Johnson's proposal fails.