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 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.
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.
The talks are stuck on the question of a "backstop" arrangement to prevent the return of border checks between Northern Ireland, part of the UK, and the Republic of Ireland if the sides fail to agree a free trade pact.
Barnier met Irish Foreign Minister Simon Coveney on Monday before heading to Strasbourg for meetings with EU parliamentarians.
"Clearly this is a very important week for Brexit negotiations. The two teams have really intensified their engagement," Coveney said afterwards.
He added: "We'll have to wait to see where that goes. The issues aren't new. We need to give the teams the time and space now."
There were signs of compromise last week on the backstop, with British concerns focused on how it might exit the arrangement in future.
But hints that May was giving ground drew the fury of her Northern Irish allies, the Democratic Unionist Party, and of eurosceptics in her own party.
The DUP, which props up her government, fears Northern Ireland will be left partially under EU regulation, diluting UK sovereignty.
Can't rush it
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 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 (45 billion euros, $50 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 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 May's proposed withdrawal deal a "terrible mistake" and calling for a second Brexit referendum.