France has threatened to veto a post-Brexit trade deal as Downing Street admitted eleventh hour negotiations were at a "very difficult point".
France's European Affairs Minister Clement Beaune threatened to block a deal if the French did not believe it was a good agreement.
Hopes of reaching an agreement were dealt a fresh blow on Thursday when the UK accused the EU of making fresh demands at the last minute.
If a deal is not struck before the end of the Brexit transition period in December, the UK faces trading on tariff-heavy World Trade Organisation terms - effectively a no deal Brexit.
Negotiators were locked in talks again on Friday, with the latest deadlock thought to centre on fishing, the “level playing field” to ensure no side undercuts the other, and the oversight of arrangement.
Mr Beaune, a close ally of French President Emmanuel Macron, warned that his country could block a deal.
"I want to tell our fishermen, our producers, the citizens who are listening that we will not accept a deal with bad terms," Mr Beaune told Europe 1 radio.
"If a good agreement cannot be reached, we will oppose it.
"Each country has a veto right, so it is possible ... We will do our own evaluation of this draft deal, if there is one."
Downing Street said the Brexit trade talks are at a "very difficult stage" after negotiations lasted until after 11pm on Thursday night.
A spokesman for the Prime Minister said: "We are committed to working hard to try and reach an agreement with the EU and the talks are ongoing.
"There are still some issues to overcome. Time is in very short supply and we are at a very difficult point in the talks."
The spokesman would not confirm if talks would go on over the weekend.
Do you think the Government is ready for Brexit?
0+ VOTES SO FAR
Boris Johnson will work from his Chequers retreat over the next few days, where he will be kept updated on the state of the talks.
Negotiations were said to be showing promise this week but then seemed to stall on Thursday.
A senior Government source said: "A breakthrough is still possible in the next few days but that prospect is receding."
On Friday, EU Council President Charles Michel said the coming hours and days would determine the outcome of discussions.
He warned: "If one side of the table rejects (an agreement), it's a no-deal. We will need to assess what will be probably on the table.
"The real question is, 'Which political, economic, social project do they want for their own future?' And this is a question for the British Government and for the British people."
He said it was “unfortunate” talks were taking “longer than planned, but we're still currently negotiating”.
He added: "We'll see over the next few days what the next steps are at this point in the negotiations."
Mr Michel claimed the EU “want a deal but not at any price”, saying: "For the European Union ... the 'level playing field' is key."
It comes as Mr Johnson was due to press ahead with controversial legislation which would break international law by tearing up parts of his own Brexit deal.
Ministers will ask MPs to reinstate controversial legislation that ignores provisions in the Withdrawal Agreement relating to Northern Ireland.
The Commons will vote on the UK Internal Market Bill on Monday, potentially throwing the trade talks into crisis.