French President Emmanuel Macron on Sunday said Britain must give ground in a post-Brexit fishing dispute or France will introduce trade reprisals this week.
"The ball is in Britain's court," Mr Macron said after meeting British Prime Minister Boris Johnson on the sidelines of the G20 summit in Rome.
The dispute over fishing rights is threatening to turn into a full-blown trade war.
Mr Macron's office said the leaders agreed to work on "practical and operational measures" to resolve the dispute in the coming days, and were united on the need for a "de-escalation".
But Downing Street denied any such agreement, and insisted it was up to Paris to back down on a threat to start trade reprisals against British fishing catches and other goods from Tuesday.
"If the British make no movement, the measures of November 2 will have to be put in place," Mr Macron said. He said he "hoped there would be a positive response tomorrow".
"I want no escalation but we must be serious," Mr Macron said.
Mr Johnson said Britain and France were "longstanding allies and friends", but: "On fish, I've got to tell you, the position is unchanged."
France is angry that Britain and the Channel Islands of Jersey and Guernsey have not given some French boats licences to fish in their waters since Brexit took full effect at the start of 2021.
Paris has pledged that unless licences were approved, it would ban UK boats from unloading their catches at French ports from next Tuesday and impose checks on all products from the UK.
"If the French government wants to come forward with proposals to de-escalate the threats they have made, we would absolutely welcome them," Mr Johnson's spokesman said earlier on Saturday.
And he said the UK would continue to process applications by French and other EU fishing vessels based solely on technical data.
The feud has already led to a British trawler being detained in a French port and Paris's ambassador in London being summoned to the Foreign Office for a dressing down.
The dispute has taken time from both leaders' packed schedules when they are working hard on the climate change agenda for the UN's Cop26 meeting, which began in Glasgow on Sunday.
Mr Johnson's focus in the meeting with Mr Macron was on persuading the EU to amend a post-Brexit protocol governing trade between Britain and its province of Northern Ireland, his spokesman said.
Mr Johnson said he was "puzzled" by a strongly worded letter from French Prime Minister Jean Castex, urging EU Commission President Ursula von der Leyen to punish Britain over Brexit.
"I don't believe that is compatible with either the spirit or the letter of the Withdrawal Agreement or the Trade and Co-operation Agreement," he said, referring to UK-EU accords.
On Saturday in Rome, Mr Johnson complained to Ms von der Leyen that the French threats were "completely unjustified".
London is warning it could activate a Brexit dispute measure for the first time, exposing all of the EU to UK trade measures if France acts on Tuesday.