The prime minister has sought to soothe relations between France and the UK, telling journalists en route to the G20 in Rome that France is one of the UK’s “best, oldest, closest friends and allies”.

Asked what he would say to President Macron when the pair meet in Rome this weekend, Mr Johnson responded: “[The] ties that unite us, that bind us together, are far stronger than the turbulence that currently exists in the relationship.

“That is what I will say to Emmanuel, who has been a friend for many years.

“And what I will also say is that there may be people on either side of the Channel that they think they have an interest in promoting disharmony between the UK and France and creating the impression of disharmony. I don’t think Emmanuel shares that perspective.”

On the issue of retaliation, the prime minister said he was “puzzled about what was going on, and we fear there may be a breach of the trade and cooperation agreement” and “we will stand by to take the appropriate action”.

Asked if the UK would back down on fishing licences, the PM said “we will do whatever is necessary to ensure UK interests” are protected.

Mr Johnson said he would be “surprised” if the French snarled up trade across the Channel.

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“I haven’t heard that from our French friends, I would be surprised if they adopted that approach.”

The PM told reporters that UK fishermen should be confident in going about their lawful business, wherever that will be.

His comments come after Brexit minister Lord Frost threatened to retaliate in the escalating row with “practical responses”.

Lord Frost said he made clear to EU Brexit negotiator Maros Sefcovic if France carried through with “unjustified” threats to disrupt fisheries and hauliers next week the UK would react “accordingly”, in both physical and legal ways.

The pair were holding talks in London about the Northern Ireland Protocol but the row over licences for French boats to fish in UK waters dominated as it took a turn for the worse yesterday when France seized a British scallop trawler and today charged its captain.

If the UK does not grant licences for 55 French vessels, France has said from next Tuesday it will block its ports, carry out security checks on British vessels, reinforce controls of lorries to and from the UK, reinforce customs and hygiene controls, and raise tariffs.

There has also been a threat of halting electricity to the Channel Islands, which are British dependencies but are close to the French coast.

A UK government spokesman said Lord Frost “made clear” to Mr Sefcovic if those threats are carried out the EU would be in breach of the trade and co-operation agreement (TCA) between the UK and the bloc, which came into force fully on 1 May.

He added: “The government is accordingly considering the possibility, in those circumstances, of launching dispute settlement proceedings under the TCA, and of other practical responses, including implementing rigorous enforcement processes and checks on EU fishing activity in UK territorial waters, within the terms of the TCA.”

Earlier, environment minister George Eustice told Sky News the UK has issued post-Brexit licences to 1,700 vessels, including 750 French fishing boats, which amounts to 98% of applicants.

He said the remaining 55 vessels, despite the UK trying to help them with the data, could not prove they had fished in Jersey’s waters previously so could not get a licence under the trade and co-operation agreement with the EU.

Mr Eustice also told Sky News: “If they [the French] do bring these measures into place, well, two can play at that game and we obviously reserve the ability to respond in a proportionate way.”

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