Boris Johnson has vowed to hit back if France breaks “international law” after Paris threatened to impose tougher customs checks or even ban British seafood in the EU over a row over access to UK waters for French fishing boats.

The dramatic intervention from Downing Street followed claims from Paris that they would retaliate over the unfair treatment of French fishing vessels seeking post-Brexit access to UK waters.

A UK government spokesperson said: “France’s threats are disappointing and disproportionate, and not what we would expect from a close ally and partner.

“The measures being threatened do not appear to be compatible with the trade and cooperation agreement (TCA) and wider international law, and, if carried through, will be met with an appropriate and calibrated response. We will be relaying our concerns to the EU Commission and French government.”

The French government has been infuriated by the response of the authorities in the UK and Jersey to post-Brexit applications from French fishing vessels to permits to its waters.

Earlier this week, the European Commission said the UK government had approved 15 out of 47 applications for French boats to operate in the so-called 6-12 mile zone from the British coast.

A further 15 applications are being considered where evidence of activity in those waters is limited, but 17 applications have been withdrawn by French applicants because of “poor evidence”.

Of greater concern to the French authorities, a third of boats applying to fish in the waters off Jersey, a British crown dependency, have also been turned down by the island’s government.

France has been consistently pushing the EU to take a stronger stance against the UK over its concerns that Boris Johnson’s government is acting in breach of its obligations over post-Brexit fishing access to Channel waters.

On Wednesday, a government spokesperson in Paris said France was drawing up a list of sanctions of which some would come into effect early next week unless enough progress had been made.

“Our patience is reaching its limits,” the spokesperson said.

“It’s a first series of measures,” said the French EU affairs minister, Clement Beaune said later. “Either this first series of measures leads to a dialogue about the licences, then that’s good. Or these measures do not lead to the deal being implemented and we will take other measures, including on the supply of electricity, for example.”

“Our objective is not to impose these measures, it is to get the licences,” Beaune added.

French ministers have in recent days threatened to cut off energy supplies to the island. Jersey has awarded 95 licences to French fishers and is giving a further 75 fishers until mid-November to supply sufficient evidence they were genuine and had fished for 10 days during the past three years.

EU sources admitted, however, that it was yet to be seen whether the applications from French fishers were valid and whether unilateral action from Paris was legal. “If it’s not an EU action, can the French actually do anything?” one source in Brussels said. “They’re in a state of ignorance. [The French president] Emmanuel Macron’s position is beleaguered.”

Under the post-Brexit deal on fishing, EU fishers seeking to access British seas had to apply for new licences that would be granted providing they could prove that they had worked in British waters in previous years.

In total, the UK has granted almost 1,700 licences to EU boats to fish in waters classed as being part of its exclusive economic zone, which stretches between 12 and 200 nautical miles from the coast, equating to 98% of applicants.

The irony of the UK accusing an EU member state of seeking to break international law was not lost in Brussels. Johnson admitted to doing precisely the same last year through its unilateral measures in relation to Northern Ireland’s post-Brexit arrangements.