The already slim chance that a Brexit deal would be agreed this month receded still further on Monday as another EU meeting ended without a breakthrough.

European Union chief negotiator Michel Barnier warned ministers from member states that no deal has been sealed yet on Britain’s departure from the bloc.

In London, Prime Minister Theresa May’s spokesman said talks in Brussels went on overnight until the early hours of Monday and would continue, but he warned: “There are still substantial issues to overcome.”

A government source said a deal must happen by Wednesday if there is any chance of an extraordinary EU summit this month to sign the withdrawal agreement.

If there is no deal in November, the crisis could drag on until a regular EU summit on December 13.

This would dramatically curtail the time May will have to get any agreement past a rebellious British parliament before Brexit day on March 29.

Talk of a Brexit deal getting closer has unleashed a fresh torrent of domestic opposition to the British prime minister’s plans, a reminder that any divorce may still be rejected in parliament.

Negotiations have been complicated throughout by the need to appease hardliners in Theresa May’s Conservative party and her Northern Irish allies, who are alert to any hint of compromise.

This weekend, a leading Brexit-backing Tory MP and a member of the Democratic Unionist Party (DUP) signed a joint letter demanding no concessions on Northern Ireland’s status in the UK.

“If the government makes the historic mistake of prioritising placating the EU over establishing an independent and whole UK, then regrettably we must vote against the deal,” Steve Baker and Sammy Wilson wrote in the Sunday Telegraph.

As fears rose that the saga will end with Britain crashing out without a deal, the pound dropped to 1.283 dollars against 1.297 at Friday’s close.

Briefing the 27 other EU member states, “Michel Barnier explained that intense negotiating efforts continue, but an agreement has not been reached yet”, the European Council of member states said.

“Some key issues remain under discussion, in particular a solution to avoid a hard border between Ireland and Northern Ireland,” it added in a statement.

EU ministers set a cautious tone as they arrived for the briefing by Barnier. “We have time but not so much,” Belgian Foreign Minister Didier Reynders told reporters.

“It’s difficult to make real progress, but before Christmas I’m hoping it’s possible,” he said, as the EU awaits a “positive signal” from London.

Britain is keen to get a deal with the EU and a provisional sign-off by the House of Commons before Christmas, to allow time for it to be put into legislation.

May will host a weekly cabinet meeting on Tuesday, but the UK source said: “As it stands there’s no agreement for them to discuss.”

The agreement is meant to finalise Britain’s exit bill of around £39 billion ($A70 billion), guarantee citizens’ rights and launch a 21-month transition, during which London will follow EU rules.

In the transition period, British and European negotiators will launch more ambitious talks to agree the future trading and legal relationship.

But May is facing increasing pressure from all sides in her Conservative party over the mooted plan.

Eurosceptics have long warned against staying too close to the EU, and former foreign minister Boris Johnson, who quit in July in protest at May’s approach, said on Monday she was “on the verge of total surrender”.

There were also signs that pro-Europeans are gearing up to block the Brexit deal after Johnson’s brother Jo, a junior transport minister, quit on Friday branding Mrs May’s proposed withdrawal deal a “terrible mistake” and calling for a second Brexit referendum.