BRUSSELS -- With a deadline looming ever more menacingly, the European Union's chief negotiator on the post-Brexit trade deal with Britain Wednesday publicly blamed London for a lack of progress in the two sides' belated attempt to reach an even rudimentary agreement.

鈥淎t this stage, there are still too many difficulties remaining on important topics,鈥 Michel Barnier said on his way to brief the envoys of the 27 member states.

In a Twitter comment later, Barnier said that 鈥淒espite EU efforts to find solutions, very serious divergences remain.鈥

Britain's chief negotiator David Frost said he agreed that 鈥渨ide divergences remain on some key issues.鈥

鈥淲e continue to work to find solutions that fully respect U.K. sovereignty,? Frost tweeted.

Barnier's comments threw a dampener on optimistic reports that progress was being made at a rapid pace on issues such as fisheries rights, one of three remaining major topics that need a compromise solution if a deal is to be found before Jan. 1, when a transition period in the Brexit divorce proceedings ends.

Barnier's stern words were in complete contradiction to the olive branch he offered London only two weeks ago after British Prime Minister Boris Johnson insisted that the EU and Barnier fundamentally had to change tack to continue the negotiations.

Both sides have been intensely negotiating since, but, as Barnier pointed out, to little effect.

The lack of progress on fisheries and on the need to have common regulatory standards and fair competition to make sure Britain won't undercut EU businesses has befuddled the negotiating teams for months, as both sides have been trying to strike a trade deal since the U.K. left the EU on Jan. 31.

They must do so within weeks if an agreement is to be ratified by the end of the year, when a post-Brexit transition period ends.

And without a deal, trade between both sides would fundamentally change, and hundreds of thousands of jobs would be threatened on both sides, especially in nations close to Britain such as Ireland, France, Belgium and the Netherlands. Barnier insisted again Wednesday that the EU 鈥渋s prepared for all scenarios.鈥

In the trade negotiations, Britain wants to retain as many of the advantages of EU membership as possible without having to live by the bloc's rules. The EU is insisting on stringent trade regulations to avoid having a giant buccaneering trade partner on its doorstep that could freely undercut the bloc's state aid, social and environmental standards.

After negotiating in Brussels this week, the talks are set to move to London again in the coming days.

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Danica Kirka in London contributed.