Two Royal Navy vessels have begun patrolling the waters off Jersey after Boris Johnson dispatched them in a deepening row between the UK and France over fishing rights.

The craft — HMS Tamar and HMS Severn — are in crowded waters as more than 50 French fishing boats converged on the port of St Helier, Jersey’s capital, on Thursday to protest against the island’s restrictions on French fishermen.

Demands that French ships be equipped with monitoring devices and meet other criteria to obtain licences prompted France to threaten to cut off its power supply to the Channel island over the dispute.

The disagreement comes amid more general complaints from French fishermen over difficulties receiving the necessary licences to fish in British coastal waters, an issue addressed by the EU-UK trade deal struck last year. Brussels has warned Britain that applying conditions without prior warning breaches this trade deal.

A spokesperson for the European Commission said that Brussels has “clearly indicated” to Britain that rights enshrined in the two sides’ trade deal “have not been respected”. The trade agreement, which underpins the new EU-UK relationship, sets out terms for access to fishing waters. “Until the UK authorities provide further justifications on the new conditions [for licences], these new conditions should not apply,” the spokesperson said.

Downing Street on Wednesday evening said Johnson had spoken to Jersey’s chief minister and external relations minister on the threat of a blockade of St Helier by French fisherman.

“The prime minister underlined his unwavering support for Jersey. He said that any blockade would be completely unjustified,” a spokesperson said. “As a precautionary measure the UK will be sending two offshore patrol vessels to monitor the situation.”

Fishermen from France’s Normandy coast say they have worked in these waters for years but now face unreasonable curbs in the aftermath of Brexit.

Dimitri Rogoff, who heads the Normandy regional fishing committee, said the idea was not to blockade St Helier but to make a point.

Carteret fisherman Camille Lécureuil told AFP, the French news agency, that the protesters had decided on Thursday morning to stop a cargo vessel, the Commodore Goodwill, from leaving the port, although the plan was for the boats to return to France in the afternoon.

“It’s a peaceful movement, there’s no need for things to get out of hand,” he said. “We even have support from Jersey. Three fishing boats from the island have come to support us.”

Brussels has the option of opening formal dispute settlement procedures with the UK under the terms of the two sides’ trade deal should the EU decide that Britain is violating the spirit of the agreement, which preserved some EU fishing rights in waters close to the Channel Islands.

UK naval vessels patrol the waters off Jersey

Jersey, the largest member of the archipelago and a British crown dependency, receives 95 per cent of its electricity from France through underwater cables. Its foreign policy is governed by the UK, which means it is treated as a third country by the EU.

Annick Girardin, the French maritime minister, told France’s National Assembly on Tuesday that she was “revolted” that Jersey had granted 41 fishing licences that included conditions and specific criteria that were “decided unilaterally and without explanation”.

“It’s unacceptable,” she told lawmakers. “We’re ready to resort to retaliatory measures . . . concerning Jersey, I’ll remind you of the transport of electricity via submarine cables.” Girardin added she would “regret” any action but “we’ll do it if we have to”.

French fishermen and ministers have been complaining for two weeks about the difficulty of gaining access to British waters despite the agreement on fisheries reached at the end of last year.

Clément Beaune, France’s junior minister for European Affairs, last week threatened to block regulations that would allow UK financial firms to do business in the EU if Britain did not respect its Brexit commitments on fishing.

Bertrand Sorre, an MP for President Emmanuel Macron’s governing La République en Marche party, shared the example of a fisherman from Granville in Normandy. The man had previously fished for scallops and whelks for an average of 40 days a year off Jersey but he had been told he could fish for only 11 days this year, and only for scallops.

Ian Gorst, Jersey’s external relations minister, said the island had issued the licences in accordance with the UK’s trade and co-operation agreement with the EU and the new regime would “take time for all to adjust”.

The UK’s Department for Environment, Food and Rural Affairs said: “We are clear that Jersey is responsible for its own territorial waters.”