Michel Barnier said that the UK has refused to give guarantees on open and fair competition after Brexit, adding that London has not engaged with the EU on key conditions.
"I came back from London this morning. We did not see any change in the position of the UK. I am worried and I am disappointed," Barnier said.
The UK has not shown any willingness to seek a compromise on fisheries, while without a long-term, fair agreement on fisheries, there will be no new trade partnership with the UK, he added.
According to Barnier, a level playing field is the only way to start a new EU/UK relationship.
"How can we conclude our long-term economic partnership agreement ... without knowing which system of state aid or subsidies the UK will put in place, without any assurance that the UK will not use its new regulatory autonomy to distort competition with us in the future?" Barnier said.
Recently, Michel Barnier announced plans to write a book on the UK's withdrawal from the bloc, slated for next year, as he has kept relevant notes throughout the four-year Brexit talks.
After Brexit was delivered on 31 January, the UK and the EU entered an 11-month transition period to set the rules for their future bilateral trade.
If no deal is secured by 31 December and talks are not extended, trade between will have to trade on the terms set by the World Trade Organization, meaning UK goods will be subject to tariffs.