EU negotiator Michel Barnier will be in London after a round of face-to-face discussions ended in Brussels earlier this week, as the parties' approaches varied.
Barnier said after the negotiations ended last Thursday that "serious differences remain".
His British counterpart David Frost said that there were "significant differences", which meant that the sites were still looking for "basic principles underlying an agreement".
And Chancellor Angela Merkel said when she took over the help of the rotating EU presidency on Wednesday that both her country and the 27-nation bloc "should prepare for the event that no agreement is reached".
Britain followed the results of a 2016 EU accession referendum and officially withdrew from the bloc after almost half a century in January.
However, a transition period to a standstill that ends on December 31st allows the UK to function effectively as if it were still a member.
Meanwhile, London and Brussels are expected to agree new trade terms that will prevent ties to the World Trade Organization's minimum standards – and the associated high tariffs and quotas – from returning.
British companies fear this possibility and want Prime Minister Boris Johnson to give them guidance as soon as possible on whether a trade agreement is feasible or not.
This would give them the opportunity to trigger costly contingency planning aimed at disrupting trading and doing business as little as possible.
However, EU officials feel much less pressured to reach a quick settlement and suggest that this could still be done by the end of October.
Brussels has shaken off Johnson's repeated threats to leave and accept very distant ties to the bloc, which made trade difficult but gave Britain greater independence.
The differences between the pages remain large.
London refuses to accept the jurisdiction of the European Court of Justice in trade disputes and wants a much larger proportion of the fishing waters it now shares with the bloc.
A separate dispute concerns the extent to which the UK must comply with EU state aid rules for key economic sectors, as well as labor and environmental regulations.
London argues that the whole purpose of leaving the block was to give Britain a greater say in its own affairs.
Brussels counters that Britain cannot expect to undercut the price block with looser standards, but still expects a favorable trade agreement.
The talks in London take place in an intensified format that follows the discussion of the video conference at the European climax of the coronavirus crisis.