Theresa May has met EU officials as the two sides scramble to finalise a Brexit deal in time for Sunday's summit of European leaders.
The EU is in a race against time to complete the text of its declaration on future relations with the UK, amid concerns from several member states.
Contentious issues include fishing rights in British waters and Gibraltar.
The European Commission said "very good progress" had been made at Wednesday's meeting but work was continuing.
The PM, who is under pressure from her own MPs not to give any further ground, held talks lasting about an hour with European Commission president Jean-Claude Juncker.
Mr Juncker has cancelled a two-day trip to the Canary Islands, on Thursday and Friday, to deal with "the many important events taking place at the moment".
European Commission Vice-President Valdis Dombrovskis said "sherpas" - officials tasked with doing the detailed work ahead of summits - were due to meet on Friday to work on the final texts of the withdrawal agreement and the future relationship.
"Not there yet" on political declaration on future UK/EU relationship, says Commission VP @VDombrovskis. Says A50 special summit needs final version and sherpas have to see it before meeting on Friday morning. Subtext: no text, no summit? (Which is the German view.)— Adam Fleming (@adamfleming) November 21, 2018
German Chancellor Angela Merkel said: "Both documents need to be ready by Sunday so that we can sign the exit agreement and accept the declaration on the future relationship."
Asked if the summit could be cancelled, Downing Street said the agenda had been published and it "looked forward to attending".
In other developments:
Before heading for Brussels, Mrs May came under fire from every Brexit faction in the House of Commons at a noisy Prime Minister's Questions - from those who want another referendum, to those who want Britain to leave the EU without a deal.
Labour leader Jeremy Corbyn zeroed in on comments by new work and pensions secretary Amber Rudd, who said MPs would prevent a no-deal Brexit, apparently putting her at odds with the PM's "no deal is better than a bad deal" stance.
He asked: "Does the prime minister agree there are no circumstances under which Britain would leave with no-deal?"
Mrs May replied "no" and said the alternative to her deal would "either be more uncertainty, more division or", in what looks like the emerging new emphasis from her government, "it could risk no Brexit at all".
Mr Corbyn said that "if the government can't negotiate an alternative then it should make way for those who can and will".
Mrs May replied: "He is opposing a deal he hasn't read, he's promising a deal he can't negotiate, he's telling Leave voters one thing and Remain voters another - whatever (Mr Corbyn) will do, I will act in the national interest."
Mrs May also rejected a call from the SNP's Ian Blackford to renegotiate her Brexit deal to keep the UK in the single market and customs union, saying it would "frustrate the vote of the British people".
And she branded Green MP Caroline Lucas's call for another referendum, on the grounds that public opinion had shifted since the 2016 referendum, "absolutely ridiculous," saying the public had given this Parliament "an instruction" to leave the EU.
Mrs May appears to have faced down the threat of a challenge to her position from Brexiteer critics of the deal, for the time being at least.
However, Tory MPs unhappy with Mrs May's handling of Brexit negotiations want much more clarity on the terms of the UK's future co-operation with the EU if they are to back the final deal.
All sides in the Commons have warned of a "blind Brexit" in which the UK signs up to a series of legally-binding commitments in the draft withdrawal agreement, without similar guarantees over future trading arrangements.
The withdrawal deal was agreed in principle by both Mrs May and the EU last week.
It includes a £39bn "divorce bill" and the controversial customs "backstop" which keeps the UK temporarily in the EU customs union as a way of preventing the return of manned customs posts at the Irish border.
However, the joint political declaration on future relations - still being drafted - would only set out the shape of the UK's trading relationship with the remaining 27-nation bloc, without any legal commitments.
Any binding trade deal would still have to be thrashed out in the 21-month transition period after Britain leaves the EU on 29 March 2019.