Talks to reach a post-Brexit trade deal are in a "serious situation", Boris Johnson said after a call with the EU Commission head Ursula von der Leyen, APA reports citing BBC.
He warned that "time was short" and that a no deal scenario was "very likely" unless the EU position changed "substantially".
Mrs von der Leyen said it would be "very challenging" to bridge the "big differences", particularly on fish.
However, she also welcomed "substantial progress on many issues".
Talks in Brussels will continue on Friday, with two weeks to go before the UK leaves EU trading rules.
In a statement issued after the phone call, No 10 said: "He [Mr Johnson] said that we were making every effort to accommodate reasonable EU requests on the level playing field, but even though the gap had narrowed some fundamental areas remained difficult.
"On fisheries he stressed that the UK could not accept a situation where it was the only sovereign country in the world not to be able to control access to its own waters for an extended period and to be faced with fisheries quotas which hugely disadvantaged its own industry.
"The EU's position in this area was simply not reasonable and if there was to be an agreement it needed to shift significantly."
The UK's chief negotiator David Frost echoed the prime minister's tone, tweeting: "The situation in our talks with the EU is very serious tonight. Progress seems blocked and time is running out."
European Parliament leaders have set Sunday as a deadline for them to see the text of any deal agreed by the negotiating teams.
The senior MEPs said they would "not be rushed" into approving an agreement at their end, and would have to see the text by the end of the week if they were to sign it off by 31 December.