Boris Johnson has refused to rule out triggering the dispute mechanism clause in the Brexit trade agreement after suggesting France had breached the terms of the deal with its fishing threats.

The Prime Minister said he was “worried” Paris “may be about to become in breach, or is already in breach” of the free trade deal agreed between the UK and the European Union, and left the possibility of escalating the issue on the table.

Asked by Sky News before his G20 meetings in Rome whether he was ruling out triggering the deal’s dispute resolution mechanism, Mr Johnson replied: “No, of course not. I don’t rule that out.”

The dispute process would see a consultation period started, after which, if there is no solution found, an arbitration panel would be formed with compensation demanded or even the treaty suspended as punishment, according to the Commons Library.

It comes after French president Emmanuel Macron, in an interview with the Financial Times, accused the UK of not keeping its Brexit pledges on fishing as he said London lacked “credibility”.

The country’s prime minister Jean Castex, meanwhile, has written to European Commission president Ursula von der Leyen to encourage Brussels to back Paris’ position against London.

France is threatening to block British boats from its ports and tighten checks on vessels if an issue over a lack of licences for small French vessels to fish in British waters is not resolved by Tuesday.

Mr Castex urged the EU to use the “levers at its disposal” to press home the need for “compliance” with the Brexit agreement on fishing and to show that “leaving the Union is more damaging than remaining in it”.

But Jean-Marc Puissesseau, president and chairman of the ports of Calais and Boulogne-sur-Mer, described the argument as “ridiculous” as he urged both sides to dial down talk of threats and retaliation.

Mr Puissesseau encouraged the Prime Minister and Mr Macron to find “agreement” when they talk on the margins of the G20 summit this weekend, with suggestions they could meet on Saturday.

He told BBC Radio 4’s Today programme: “It will be terrible for both sides of the Channel: for you, for us, for the ports, the fishermen in your country, for the fishermen in our country.

“And that’s only for 40 little boats which are not allowed to fish in your country, so I hope there will be an agreement on that over the weekend.”

In Rome, Mr Johnson left the door open to finding a resolution with Mr Macron as he described the UK as being “very keen to work with our friends and partners”.

He repeated the assertion he made on the plane over to Italy, vowing to “take steps to protect UK interests” if there is a breach of the Trade and Co-operation Agreement (TCA) with the EU.

Asked whether he thought France had breached the trade agreement, the Prime Minister told Sky News: “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.”

In a meeting with Ms von der Leyen in the Italian capital, Downing Street said the Prime Minister warned that Paris’ threats did “not appear to be compatible with the UK-EU TCA or wider international law”.

But, according to No 10, he stressed that the UK was “happy to consider any further evidence” for licence applications which had been rejected. The UK has said only 2% of EU vessel applications had been denied.

In comments made before the Rome talks, a commission spokesman said the trade deal made provision that “vessels who were fishing in these waters before (Brexit) should be allowed to continue” and implored Britain to look at each case, “boat-by-boat”.

The wrangle over fishing access escalated this week after French authorities accused a Scottish-registered scallop dredger of fishing without a licence.

The captain of the Cornelis Gert Jan vessel, understood to be an Irish national, was detained in Le Havre during the diplomatic storm and has been told to face a court hearing in August next year.

French authorities allege the Cornelis Gert Jan did not have a licence, a claim the boat’s owner, Macduff Shellfish, denies. The EU said UK authorities withdrew the licence on March 1.

Foreign Secretary Liz Truss took the rare step of ordering an allied nation’s envoy to be summoned as she called Catherine Colonna, French ambassador to the UK, to the Foreign Office on Friday afternoon to challenge her over France’s stance.

The meeting came after ministers promised retaliation if France did not back down over its proposals if next week’s deadline for increased licences is not met.

At the centre of the dispute are the licences for small boats, which are issued only if the vessels can demonstrate a history of fishing in British waters.