Michel Barnier has hit out at the lack of progress in the Brexit negotiations and said the "clock was ticking".
The European Commission's chief Brexit negotiator said the EU "respects" the UK's timetable but Britain cannot both slow down trade talks on key areas while refusing to agree to extend the transition period.
Mr Barnier accused the UK of not commiting "seriously" on numerous points from the political declaration and said only "very" partial progress had been made in the negotiations.
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.
He said his mandate was to get "tangible progress", adding: "I'm afraid this has only been very partially met this week.
Speaking at a Brussels press conference, Mr Barnier said: "The UK has affirmed once again this week its wish to make tangible progress between now and June and we're on the same wavelength on this and we respect the same timetable.
"That means that we need genuine progress by June if, at the end of this year, we want to strike an agreement which is commensurate to the level of our economic interdependence and geographical proximity."
He added: "We have to go beyond clarification stages, we need more politically dynamic process-generating proposals to build steps towards a compromise."
Despite warnings an agreement may not be possible by the end of the year when the transition ends, Boris Johnson has insisted he would not agree to an extension.
Mr Barnier said there are four areas in particular where the "progress this week was disappointing", including the level playing field, fisheries and the role of the European Court of Justice.
Those were agreed by the Prime Minister and his team in the political declaration developed by both sides, which he said must be implemented in a "serious, objective, legal way".
"This is not the case now yet in a number of areas. I regret that and it worries me," he added.
"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."
A Government spokesperson said that the UK regrets that "limited progress" has been made in bridging the gap between the two sides.
They said in a statement: “This was a full and constructive negotiating round, conducted remotely by video conference, and with a full range of discussions across all the issues, on the basis of the extensive legal texts provided by both sides in recent weeks.
“However, limited progress was made in bridging the gaps between us and the EU.
“Our assessment is that there was some promising convergence in the core areas of a Free Trade Agreement, for example on goods and services trade, and related issues such as energy, transport, and civil nuclear cooperation. “We regret however that the detail of the EU’s offer on goods trade falls well short of recent precedent in FTAs it has agreed with other sovereign countries. “This considerably reduces the practical value of the zero tariff zero quota aspiration we both share.
“There are also significant differences of principle in other areas. For example we will not make progress on the so called "level playing field" and the governance provisions until the EU drops its insistence on imposing conditions on the UK which are not found in the EU’s other trade agreements and which do not take account of the fact that we have left the EU as an independent state.
“On fisheries, the EU's mandate appears to require us to accept a continuance of the current quotas agreed under the Common Fisheries Policy. We will only be able to make progress here on the basis of the reality that the UK will have the right to control access to its waters at the end of this year.
“We now need to move forward in a constructive fashion. The UK remains committed to a deal with a Free Trade Agreement at its core. We look forward to negotiating constructively in the next Round beginning on 11 May and to finding a balanced overall solution which reflects the political realities on both sides.”
