The escalation in tensions in the English Channel between France and the UK over fishing rights may in fact be a symptom of a wider dispute over Brexit, with French Prime Minister Jean Castex admitting that Britain should be punished for leaving the European Union.

In a leaked letter to European Commission President Ursula von der Leyen concerning the ongoing fishing rights disputes, Mr Castex said that Brussels needs to make an example out of Britain to show that leaving the EU is “more damaging than remaining”.

In the letter, obtained by POLITICO and seen by other news outlets, the French PM wrote that it is “necessary for the European Union to show its full determination to obtain full compliance with the agreement by the United Kingdom and assert its rights by using the levers at its disposal in a firm, united and proportionate manner.

“It is essential to make clear to European public opinion that compliance with the commitments entered into is non-negotiable and that leaving the Union is more damaging than remaining in it.”

Mr Castex went on to argue that if Britain fails to meet the demands from Paris on the number of fishing licenses granted, then the EU should trigger article Article 506 of the Brexit agreement in order to impose sanction-like tariffs on British seafood. The French government has already threatened to take such actions unilaterally, however, has so far seen minimal support in Brussels.

France has accused the British government of withholding fishing licenses for French boats to fish in Britain’s lucrative fisheries in the English Channel and off the coast of Jersey. The UK, for its part, has claimed that the vessels denied licences did not have a history fishing the waters in question prior to Brexit and therefore should not be granted access following Britain’s official departure from the EU.

In response, France has threatened to use time-consuming border checks on British products in order to exacerbate supply chain issues. The French government has also hinted at a blanket ban on British seafood from being traded in its ports.

Paris has previously threatened to cut off energy entirely to Jersey, the largest of the Channel Islands in the English Channel and a British Crown Dependency off the coast of France.

In a further escalation of tensions, a British fishing vessel was detained off the French fishing port town of Le Harve on Wednesday. According to a report in The Times, the captain of the boat has been charged by the French with unauthorised fishing and could face a criminal trial and fines of up to €75,000 (£63,000/$86,000).

The owner of the boat, Scotland’s Macduff Shellfish, told The Telegraph that it was operating within the law at the time of the detention, with director Andrew Brown saying that it may have come as a result of an “extremely technical rule” over engine upgrades which could have been resolved with a “phone call”.

Following the detention of the fishing vessel and the increase of “confrontational language”, Britain’s Foreign Office summoned French Ambassador Catherine Colonna on Friday.

Yet, the British government of Prime Minister Boris Johnson has taken a notably softer line in the dispute than their French counterparts.

Brexit minister Lord David Frost told the European Commission the UK could launch “dispute settlement proceedings” if France follows through with its threats.

Ahead of a meeting with French President Emmanuel Macron at the G20 summit in Rome on Saturday, Prime Minister Boris Johnson said that he will do “whatever is necessary” to defend British interests in the fishing dispute.

“We fear that there may be a breach of the terms of the trading co-operation agreement implicit in what’s happening, in some of the things that are being said, and obviously we will stand by to take the appropriate action,” Johnson said.

Brexit leader Nigel Farage has called on the government to take a harder line, suggesting that the next time a British ship is seized by the French, then the UK should “do the same” the next time a French boat enters within six miles of the British coast.

Follow Kurt Zindulka on Twitter here @KurtZindulka