Updated 28 minutes ago
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HMS Severn is one of the two offshore patrol vessels being deployed to Jersey
The UK is sending two Royal Navy patrol vessels to monitor a protest in waters around Jersey's main port amid a fishing rights row with France.
PM Boris Johnson said "any blockade would be completely unjustified" as over 100 French fishing boats prepared to sail to the island on Thursday.
It follows a threat by France to cut off Jersey's electricity over new post-Brexit rules for French fishing boats.
No 10 said sending the Navy vessels was a "precautionary measure".
HMS Severn is currently heading towards the waters off Jersey's port of St Helier. HMS Tamar will follow later.
HMS Severn has previously been used to shadow Russian navy warships off the English coast, and the ships are routinely used for fisheries protection - with sailors able to board other boats for spot checks.
On Wednesday night, Mr Johnson pledged his "unwavering support" for Jersey, the largest Channel Island and a Crown dependency, located 14 miles (22km) off France.
Earlier, he had held talks with Jersey's Chief Minister John Le Fondré and Minister of External Affairs Ian Gorst, and "stressed the urgent need for a de-escalation in tensions" between Jersey and France.
Senator Gorst told the BBC the threats from Paris were "disproportionate" but he was expecting a "peaceful demonstration" by French fisherman on Thursday morning.
He said Jersey took "a threat to blockade our harbour very seriously" but insisted the "resolution has to be through diplomacy".
Having held talks with the president of the La Manche region of northern France, Marc Lefevre, on Wednesday, Mr Gorst said there were a number of issues which "we will continue to work through".
The AFP news agency reported that about 100 French fishing vessels would sail to Jersey's port on Thursday as part of a protest against the new rules, quoting the head of fisheries for the Normandy region, Dimitri Rogoff.
Mr Rogoff said the vessels would not try to block the port at St Helier and would return to France in the afternoon, AFP reported.
The new fishing rules - introduced by the Jersey government under the UK-EU Trade and Cooperation Agreement (TCA) - require French boats to show they have a history of fishing in Jersey's waters.
But French authorities said "new technical measures" for fishing off the Channel Islands had not been communicated to the EU, rendering them "null and void".
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French fishermen have complained about being prevented from operating in British waters because of difficulties in obtaining licences.
Under an agreement with the EU, French boat operators must show a history of fishing in the area to receive a licence for Jersey's waters. But it has been claimed additional requirements were added without notice.
Jersey has the sole power to issue the licences, and as of last week all fishing boats were required to have a licence to operate there.
On Friday, the Jersey government granted 41 permits to French fishing vessels that are equipped with technology that allows them to be located.
But the French government claimed the list of approved ships came with further demands that "were not arranged or discussed, and which we were not notified about".
Chris Le Masurier, who runs Jersey Oyster and Normandy Trader Freight, said the French fisherman were rightly upset by the situation.
He said: "I see it as very much an insult to them and they are extremely upset. The criteria that they were given was to prove they have fished in Jersey waters for 10 days. Nothing about what species were caught, nothing about if you've fished for 20 days or 30 days [and having to] prove that."
But Don Thompson, from the Jersey Fisherman's Association, said affected French crews have "had since 1 January" to comply with the new rules and "perhaps some of the boats that perhaps didn't qualify are a little bit put-out".
The threat to cut off Jersey's electricity supply - 95% of which is delivered by three underwater cables from France - was made by French Maritime Minister Annick Girardin.
She told the French parliament on Tuesday that new rules governing access to Channel Islands waters were unacceptable - and that France was "ready to use... retaliatory measures".
"I am sorry it has come to this [but] we will do so if we have to," she said.
A row that has been brewing for months about fishing rights off the coast of Jersey is now making front page news.
Downing Street's insistence that it wants to see tensions "de-escalated" may be met with some raised eyebrows when, at the same time, Mr Johnson sends in two patrol boats.
The government insists it is a precautionary measure.
Regardless, it sets the stage for a potentially dramatic day on the waters around the Channel Islands.
And it is a very real demonstration of how some of those post-Brexit arrangements, that were spelled out on paper months ago, are still being worked out in practice.
The Jersey government said fishing permits must "correspond to the previous activity a vessel has carried out in Jersey waters" under the terms of the UK-EU Trade and Cooperation Agreement.
It said its new system was "in line with the data submitted by the French and EU authorities".
A spokeswoman said it took French complaints over the terms of the licensing agreement "very seriously" and would respond, but said it had acted in "good faith" setting up the regime.
"The government remains committed to the sustainable management of Jersey waters for the benefit of this and future generations," she said.
HMS Severn and HMS Tamar are based in Portsmouth. They are both 90.5m in length, have two large guns, including a short-range anti-aircraft weapon, and are crewed by 45 sailors and up to 50 Royal Marines.