France warned Tuesday it was weighing its response after the UK imposed rules governing access for French fishing boats near the Channel Islands, and said it could involve the electricity supply via underwater cables.

French fishermen also plan to converge on the island's main port St Helier on Thursday, although authorities have said they do not intend to block access.

But Johnson announced on Wednesday that he was sending two patrol vessels "as a precautionary measure", adding that a blockade "would be completely unjustified."

British MP Tobias Ellwood accused France of "shameful behaviour," saying "it would be wise to dispatch" a Royal Navy vessel.

French maritime minister Annick Girardin accused Jersey, the largest Channel Island, of dragging its feet over the issuing of licences to French vessels under the terms of Britain's post-Brexit trade deal with Brussels.

Jersey, a self-governing British Crown dependency off the coast of France, has said it will require boats to submit further details before the licences can be granted, and pleaded for patience.

Johnson spoke to Jersey Chief Minister John Le Fondre on Wednesday, when the pair "stressed the urgent need for a de-escalation in tensions," according to a statement from Downing Street.

"The Prime Minister underlined his unwavering support for Jersey," it added.

A spokesman for Johnson's government earlier called threats over Jersey's electricity supply "unacceptable and disproportionate."

- 'Optimistic' -

The deepening row over fishing is one of several disputes that have emerged between the UK and the European Union since London left the bloc's single market and customs union at the start of the year.

Jersey External Affairs Minister Ian Gorst told BBC Radio on Wednesday: "It would seem disproportionate to cut off electricity for the sake of needing to provide extra details so that we can refine the licences.

"I do think a solution can be found. I am optimistic that we can provide extra time to allow this evidence to be provided."

Paris and London have increasingly clashed over fishing in recent weeks, as French fishermen say they are being prevented from operating in British waters because of difficulties in obtaining licences.

On Thursday morning, around 100 French fishing vessels will sail to Jersey port to protest over the issuing of the licences, the head of fisheries for the Normandy region, Dimitri Rogoff, told AFP.

Rogoff said however that they would not try to blockade the port and would return to France in the afternoon.

In the latest move, Britain on Friday authorised 41 French ships equipped with Vessel Monitoring System (VMS) technology -- which allows ships to be located -- to fish in waters off Jersey.

But this list was accompanied by new demands which France's fisheries ministry has said were not arranged or discussed with Paris, effectively creating new zoning rules for the waters near Jersey.

UK government minister Nadhim Zahawi said the two sides need to work "constructively" on "operational challenges that we need to fix together".

"This is an issue for the (European) Commission to work with our team," he told Sky News.