President Emmanuel Macron has warned that if Britain doesn’t succumb to French demands on post-Brexit fishing licenses within 24 hours, then France will go forward with trade war-style threats in the English Channel.

Despite initial reports on Sunday suggesting that the United Kingdom and France had agreed to de-escalate tensions, the two sides now appear to have no intention of backing down.

France has threatened to cut off energy to the Isle of Jersey, block British fishing boats from French ports, and increase import checks on British ships and lorries, should Boris Johnson fail to grant even more licences to fish in British waters to French trawlers.

Britain, for its part, has claimed that the boats in question have not demonstrated a history of fishing in the region prior to Brexit and therefore have no right to access the lucrative fisheries.

Speaking to reporters in Rome, Macron said per The Times: “The ball is in Britain’s court,” adding that “If the British make no movement, the measures of November 2 will have to be put in place.”

Mr Johnson, who has suggested that if the French were to enact such measures then the entire European Union would be in violation of the Brexit agreement, said: “On fish, I’ve got to tell you the position is unchanged.”

The already heightened tensions were escalated further last week, after a leaked letter from French prime minister Jean Castex said that the British should be punished for Brexit, telling EU Commission President Ursula von der Leyen that Britain’s exit from the EU should be “more damaging than remaining”.

Commenting on the letter in Rome, Johnson said: “I must say I was puzzled to read a letter from the French Prime Minister explicitly asking for Britain to be punished for leaving the EU.

“I just have to say to everybody I don’t believe that that is compatible either with the spirit or the letter of the Withdrawal Agreement or the Trade and Cooperation agreement, and that’s probably all I’ll say about that one,” he added weakly.

The letter from the French prime minister has also been taken as confirmation of long-held beliefs among some in the British government that the European Union is not negotiating in “good faith” in terms of the Northern Ireland protocol.

The agreement on Ulster (Northern Ireland) — an integral part of the United Kingdom — has seen the establishment of border checks on goods between the British province and the rest of the United Kingdom, in large part administered by the EU.

The checks have resulted in disruptions to intra-UK trade, leading to food shortages and violent protests.

Britain’s Brexit negotiator, Lord David Frost, wrote on social media that he hopes the opinion that the UK should be punished for Brexit shared in the French letter are “not held more widely across the EU” and that the UK may be forced to launch legal proceedings under the Brexit agreement.

“To see it expressed in this way is clearly very troubling and very problematic in the current context when we are trying to solve many highly sensitive issues, including on the Northern Ireland Protocol,” Lord Frost added.

On top of the threats made by the French, a British fishing vessel was detained off the French fishing port town of Le Havre on Wednesday in an apparent retaliatory strike against the UK. French courts have reportedly demanded that the trawler pay £125,000 bail in order to be released.

With Monday being a bank holiday in France, the ship will likely be stuck in the port until at least Tuesday. A crew member of the vessel told the Mail on Sunday: “We’ve been told we are not leaving this weekend. We just want to get out of here.”

The trawler, owned by Scotland’s Macduff Shellfish, has been accused by the French of not having a valid licence because of updates to the boat’s engine. The British company has denied any wrongdoing, but the captain is facing fines up to €75,000 (£63,000/$86,000).

Environment, Food and Rural Affairs Secretary George Eustice warned earlier this week that if the French carried through with threats of blockading British boats, the UK may act in turn, saying: “Two can play at that game” — although as things stand it is just the French dishing out hits, while the British government merely offers a little tough talk.

Follow Kurt Zindulka on Twitter here @KurtZindulka