Boris Johnson's dinner with European Commission President Ursula von der Leyen has ended - with a senior Number 10 source warning "very large gaps remain" between the UK and the EU on a Brexit trade deal.
"A frank discussion" was had about the "significant obstacles" that remain in the negotiations - and the source said "it is still unclear whether these can be bridged".
Mr Johnson and Mrs von der Leyen have agreed to further discussions over the coming days between their negotiating teams, and the prime minister "does not want to leave any route to a possible deal untested".
Both leaders have agreed that a firm decision about the future of the talks should be taken by Sunday.
In a statement following the three-hour meal - longer than the two hours initially scheduled - Mrs von der Leyen added: "We had a lively and interesting discussion on the state of play across the list of outstanding issues. We gained a clear understanding of each others' positions. They remain far apart.
"We agreed that the teams should immediately reconvene to try to resolve these essential issues."
Sky's Europe correspondent Adam Parsons, who is in Brussels, said a trade deal wasn't expected to emerge during the dinner - but it was hoped the two leaders would find some potential areas for compromise.
He added: "Who knows, perhaps they have - but the smoke signals coming from this are not too good. We had two separate statements. When things are going well, according to Brussels diplomats, you get one statement put out together.
"But what we do have is a deadline. By the end of this weekend, we should know whether we are heading towards no deal or a really rushed-through deal."
Some EU diplomats remain concerned that any more concessions will give the UK "too good a deal" - and European leaders are set to be briefed during a summit that begins on Thursday.
"Right now, in Brussels, it feels like no-deal is percolating up as the most likely outcome," Adam Parsons added.
Sky's Jon Craig, in Westminster, reports that attention will move to Westminster on Thursday when Michael Gove faces MPs for an urgent question from Labour in the Commons - and the prime minister may call a special meeting of his cabinet.
The prime minister had landed in Brussels just before 6pm on Wednesday night on his first visit to the Belgian capital since his landslide election win last winter.
Mr Johnson and Mrs von der Leyen agreed to hold emergency talks in an attempt to break the deadlock in trade deal negotiations.
Despite discussions going on for 10 months, three sticking points remain with only a matter of weeks until the end of the transition period: fishing, fair competition guarantees and how any future disputes should be settled.
Sky's Jon Craig, who is in Westminster, noted: "You could argue that the prime minister's Brussels hosts either had a sense of humour or were being provocative when they chose the menu.
"Fishing rights is one of the big disputes. What was he served? Scallops as a starter, and turbot for main course."
The transition period is the arrangement the UK has been in since Brexit happened.
It meant that while the country has formally left the EU and ceased to have any representatives in the European Parliament, it has still followed many of the bloc's rules.
This was intended to limit disruption to businesses, which would then only have to make a major adjustment to new regulations once - giving negotiators time to agree what the new trading relationship should look like.
But a deal has still proved elusive.
Mr Johnson is facing twin pressures from those in his own party who believe no-deal would have a severe impact on the economy, and those who believe no agreement would leave the UK better off.
Rachel Reeves, the shadow chancellor of the Duchy of Lancaster, said: "The prime minister promised an oven ready deal.
"He needs to get it done so we can focus on what matters to the British people: securing our economy, protecting our NHS and rebuilding our country."
Meanwhile, the SNP's leader in Westminster, Ian Blackford, warned: "A no deal would be a massive failure of diplomacy and leadership which @BorisJohnson has to take ownership of.
"On top of the health & economic impact of covid this is self induced self harm.
"Disruption to trade, tariffs, higher prices and lost jobs is never a price worth paying."
A new deadline - but talks may not end if there isn't agreement
By Sam Coates, deputy political editor
Finally, Boris Johnson and Ursula von der Leyen managed almost to agree on something: the creation of a new deadline on Sunday.
This looks like it could be the moment the talks are pulled if there is no agreement, and planning for no-deal becomes the sole focus of the EU and UK governments.
But even here, things are not 100% clear.
A UK source says that "by Sunday a firm decision should be taken on the future of the talks".
Von der Leyen has a subtly different take: talks should reconvene to look at "essential" issues and "we will come to a decision by the end of the weekend".
The implication is clear - but neither side has committed to ending all talks if there isn't agreement.
So perhaps even this isn't as certain as first appears. The only certainty in Brexit is the relentless ticking of time towards 1 January 2021.
