Two Royal Navy vessels are patrolling waters around Jersey amid concerns of a possible blockade by French boats over post-Brexit fishing rights.
France has threatened to cut off electricity to the island, which began issuing licences to French boats under a new system last week. Fishermen who claim their rights are being unfairly restricted are due to protest at Jersey's port of St Helier.
No 10 said it was sending two Navy vessels to "monitor the situation".
HMS Severn, which has previously been used to shadow Russian navy warships off the English coast, and HMS Tamar arrived near Jersey on Thursday morning. The ships are routinely used for fisheries protection - with sailors able to board other boats for spot checks.
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On Wednesday night, Prime Minister Boris Johnson pledged his "unwavering support" for Jersey, the largest Channel Island and a Crown dependency, located 14 miles (22km) off France.
He added that "any blockade" by French boats would be "completely unjustified".
Earlier, he 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 French threats were "disproportionate" but he was expecting a "peaceful demonstration" by 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". Dimitri Rogoff, head of fisheries for the Normandy region, said the boats would not try to block 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".