The EU must show that Brexit has been“damaging” to the UK, the French prime minister has told Brussels in a letter calling for support for tougher actions over the Channel fishing dispute.

Jean Castex wrote a letter to European Commission president Ursula von der Leyen on Thursday setting out reasons why the EU should act against Britain over the limited granting of licences for French fishermen to operate in British waters post-Brexit.

He wrote that the EU had to make clear that “leaving the union is more damaging than remaining in it”.

Paris is threatening to increase checks on British boats, stop them landing in French ports, slow customs arrangements in Calais and increase tariffs on energy bills in Jersey from Tuesday unless French fishermen are granted more licences to British waters around Jersey and Guernsey. 

A translation of Mr Castex’s letter says: “The uncooperative attitude of the United Kingdom today risks not only causing great harm to fishermen, mainly French, but also for the [European] union, in that it sets a precedent for the future and challenges our credibility and our ability to assert our rights with regard to international commitments signed by the union.

“It therefore seems necessary for the European Union to show its total determination to obtain full respect for the agreement by the United Kingdom and to assert its rights by using the levers at its disposal in a firm, united and proportionate manner.

“It is essential to clearly show to European public opinion that respect for subscribed engagement is non-negotiable and that there is more damage in leaving the union than in remaining there.”

Mr Castex continued in his letter: “If no satisfactory solution is found in this context, the European Union will have to apply Article 506 of the Trade and Cooperation Agreement and take corrective measures, in a manner proportionate to the economic and social damage resulting from the breaches.”

He suggested to Ms Von der Leyen that they could impose “customs duties on certain fishery products” as punishment.

The “restoration of trust” lies upon the UK granting French fishermen more fishing licences, his letter also says.

Boris Johnson has said that he would “do whatever is necessary to ensure UK interests” if France carries out its threats over the licences.

He said that the Anglo-French relationship was undergoing “turbulence” and that the French authorities’ decision to impound a British fishing vessel may have broken international law.

Mr Johnson will meet with French president Emmanuel Macron over the weekend at the G20 summit in Rome for their first face-to-face encounter in four months – after both sides have been accusing each other of breaching the Brexit agreement.

The PM, who arrived in Rome last night, responded to the French threats by saying: “We fear that there may be a breach in terms of the Trade Co-operation Agreement implicit in what’s happening. We will standby to take the appropriate action.”

In an interview with the Financial Times, Mr Macron suggested that the UK’s “credibility” was at stake over the dispute.

He told the newspaper: “When you spend years negotiating a treaty and then a few months later you do the opposite of what was decided on the aspects that suit you the least, it is not a big sign of your credibility.” By Lamiat Sabin, The Independent/Yahoo News