Progress in the post-Brexit talks has been "disappointing" with the UK refusing to commit "seriously" on numerous fundamental points, the EU's chief negotiator said.
Michel Barnier warned the "clock was ticking" and said Britain cannot both slow down trade talks on key areas while refusing to agree to extend the transition period.
His remarks yesterday came at the end of the second round of talks, which took place by video-conferencing technology because of coronavirus.
The UK government also acknowledged only "limited progress" was made in "bridging the gaps" with the EU.
Despite warnings an agreement may not be possible by the end of the year when the transition ends, Prime Minister Boris Johnson has insisted he would not agree to an extension.
Mr Barnier said there are four areas where "progress this week was disappointing", including the level playing field - setting common standards and rules to prevent businesses on one side undercutting the other - fisheries and role of the European Court of Justice.
He said tangible progress had been "very partially met", adding: "The UK did not wish to commit seriously on a number of fundamental points.
"I regret that and it worries me.
"We need to find solutions on the most difficult topics. The UK cannot refuse to extend transition and at the same time slow down discussions on important areas."
Under the Withdrawal Agreement struck with Brussels, the transition period in which the UK continues to follow Brussels' rules runs until the end of the year.
It can be extended if more time is needed to secure a comprehensive trade deal if a request is agreed by both sides, but that must be lodged by the end of June.
Mr Barnier criticised the UK team, led by his counterpart David Frost, for having "failed to engage substantially" on the subject of the level playing field. The EU wants a single framework to jointly manage the future relationship but the UK "continues to insist on a number of separate agreements", he said.
Britain also refused to "provide firm guarantees rather than vague principles on fundamental rights and individual freedoms" creating "serious, serious" limitations for a security partnership.
And "no progress" has been made on fisheries as the UK has "not put forward a legal text", Mr Barnier said.
A UK spokeswoman said no progress could be made until Brussels accepted "the UK will have the right to control access to its waters at the end of this year".