The UK is sending two Royal Navy patrol vessels to Jersey following threats of a blockade by French fishermen.
The move is a marked escalation in the growing row over post-Brexit fishing rights in the area.
Earlier the government reacted angrily to what it said were “disproportionate” and “unacceptable” threats after French ministers floated the possibility of cutting off the island’s electricity supply.
French fishermen are also reportedly considering plans to prevent goods entering or exiting Jersey ports in protest at the new regulations, which they say threaten their ability to make a living.
Boris Johnson has warned that any blockade would be “completely unjustified”.
As a precaution the prime minister is sending two Royal Navy Offshore Patrol Vessels to monitor the situation, Downing Street said last night.
In a phone call with the chief minister of Jersey Mr Johnson also stressed the need for an urgent de-escalation in tensions as well as dialogue on fishing access.
Foreign Secretary Dominic Raab and minister for trade Greg Hands are understood to have spoken to their French counterparts to raise concerns.
But government sources said the French had rebuffed attempts to organise a meeting between their maritime minister and the UK’s environment secretary George Eustice.
As tensions heightened the French region of Normandy also closed an office on the island.
“To threaten Jersey like this is clearly unacceptable and disproportionate,” a UK government spokesman told reporters in London.
“We are working closely with the EU and Jersey on fisheries access provisions following the end of the transition period so trust the French will use the mechanisms of our new treaty to solve problems.”
The fishermen are annoyed that, under the new post-Brexit fishing regime, they are being asked to produce copious documentary evidence proving they have previously fished in certain waters to obtain a licence to continue the activity.
They have already staged protests at French ports and stopped some British fish from landing – exacerbating problems British exporters are already facing thanks to the UK’s departure from the EU.
In a statement the government of Jersey said the French reaction “results from a misunderstanding” and that Jersey had “acted on legal advice, in good faith, and with due regard to non-discriminatory and scientific principles at every stage of these proceedings”.
As a crown dependency Jersey’s foreign affairs are ultimately the responsibility of the UK, though day-to-day relations are conducted by its own government.
Jersey’s foreign affairs minister Ian Gorst said the island wanted “to heal the relationship as soon as possible”.
“We are entering a new era and it takes time for all to adjust. Jersey has consistently shown its commitment to finding a smooth transition to the new regime, most evidently by creating an interim arrangement to allow French fishermen time to submit their data.
“That commitment remains. If French fishermen or the authorities have further evidence they would like to submit, we will update the licences to reflect that evidence.
“There is no time limit on submitting evidence, and we would like to offer French fishers the opportunity to submit data directly to Jersey, in case they feel information is not travelling quickly enough through the Normandy/France/EU/UK/Jersey route.”
Responding to questions in the national assembly on Tuesday, France’s maritime affairs minister Annick Girardin said she was “revolted” by the UK government’s approach and that France was ready to retaliate.
Asked about the question of “retaliatory measures”, the minister noted that Jersey relied on “the transmission of electricity by underwater cable” from France.
“I would regret it if we were to get there, we will do so if we have to,” she said.
The Jersey Evening Post newspaper said that the possibility of stopping all commercial vessels accessing French ports was discussed at French protests, in addition to disrupting the island’s electricity supply.
