Boris Johnson has told the European Commission president that French threats to hold up British exports in response to a dispute over fishing licences were “completely unjustified”, as the ports of Calais and Boulogne warned of an impending disaster. During a meeting with Ursula von der Leyen at the G20 summit in Rome, the prime minister raised his “concerns about the rhetoric from the French government”, a government spokesman said. The UK’s Brexit minister, David Frost, also expressed his “concern and surprise” over a letter from the French prime minister, Jean Castex, to Von der Leyen in which he sought an EU-level response on the alleged shortfall in licences for French vessels wanting to operate in the coastal waters of the UK and Jersey. Castex had written to Von der Leyen, who has yet to publicly back Paris on the issue, that it was “indispensable to show European public opinion that … it causes more damage to leave the EU than to stay in”. “I hope this opinion is not held more widely across the EU,” Frost tweeted on Saturday in response to the letter. “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.” Frost, who called for the French government to step back from its plans at its ports, also said: “This is all the more so as the threats made by France this week to our fishing industry, to energy supplies, and to future cooperation, eg, through the Horizon research programme, unfortunately form part of a pattern that has persisted for much of this year. “As I set out yesterday to [European Commission vice-president] Maroš Šefčovič, these threats, if implemented on 2 November, would put the EU in breach of its obligations under our trade agreement.” Frost said the UK government was “actively considering launching dispute settlement proceedings as set out in article 738 of the [trade and cooperation agreement]”, which could lead to the use of tariffs on French goods if arbitration failed. Paris has said it will gradually increase customs and health controls on freight, make more rigorous checks of lorries coming in and leaving France and prohibit trawlers from landing their catch in French ports if the dispute was not resolved. The French government is also considering raising the price of nuclear energy provided to Jersey through undersea cables. Jean-Marc Puissesseau, the head of the ports of Calais and Boulogne, said he had already received instructions to stop British fishers from unloading in Boulogne from Tuesday while the border authorities at Calais would enforce tougher controls on goods-laden lorries. “It will be a drama, it will be a disaster,” he told the BBC. “It will be a chaos in your country because the trucks will not cross, it will be chaos at the ports … It has reached a ridiculous point, I would say. “I hope that the British and [the French] find an agreement, we find a solution to get out of this point. I know that there is some discussions during the weekend so I do really hope.” Speaking at the G20 summit, Johnson also confirmed the UK could consider formal action under the trade and cooperation agreement. Asked if he would rule out triggering any such action next week, Johnson told Sky News: “No of course not, I don’t rule that out.” Speaking during a round of broadcast interviews at the Colosseum in the Italian capital, he said: “If there is a breach of the treaty or we think there is a breach of the treaty then we will do what is necessary to protect British interests.” Asked if he believed there had been a breach, he said this was likely but stressed his desire that the row should not overshadow efforts on climate change being discussed at the G20 ahead of next week’s Cop26 summit in Glasgow. “I am worried that there might be. I am looking at what is going on at the moment and I think that we need to sort it out but that is quite frankly small beer, trivial, by comparison with the threat to humanity that we face,” he said. The UK has only approved 16 out of 47 applications for French boats to operate in the UK’s coastal waters. A further 14 applications are being considered where evidence of activity in those waters was limited, but 17 applications had been withdrawn by French applicants because of “poor evidence”. Of greater concern to the French authorities is that 55 boats applying to fish in the waters off Jersey have been turned down by the island’s government due to lack of evidence that they have fished there for 10 days in any of the past three years. Officials from the European Commission, the UK, France and Jersey were seeking to find a way out of the crisis in talks through the weekend. In an interview with the Financial Times, France’s president Emmanuel Macron, who will meet Johnson at the G20 meeting in Rome on Sunday, said the row was a test of the UK’s credibility. “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. This article was amended on 30 October 2021 to correct the spelling of Colosseum.