Boris Johnson and European Commission President Ursula Von Der Leyen are to speak later as they try to break the post-Brexit trade deal stalemate.
The leaders were called in after negotiators for the two sides said “significant divergences” remained following a week of intensive talks.
“If there is still a way, we will see,” EU negotiator Michel Barnier said.
Sticking points include fishing rights, rules on state subsidies for business and arrangements for policing any deal.
The UK left the EU on 31 January but remains under EU trading rules until a transition period ends on 31 December.
One source close to the negotiations on the UK side suggested there had been a more optimistic outlook earlier in the week but pointed to demands for EU fishing boats to have 10-year access to UK waters as one issue that derailed progress – as had been reported in the Telegraph.
BBC political editor Laura Kuenssberg said the pattern of recent UK negotiations with the EU was for victory to be snatched from the verge of defeat at the very last moment – but that one member of the government was now putting the chances of a deal at around 50-50.
She said it would be complacent to think it would all automatically fall into place after a last bit of political scrapping.
Speaking to reporters in London as he prepared to return to Brussels, Mr Barnier said: “We keep calm, as always, and if there is still a way, we will see.”