Two Royal Navy ships are patrolling waters around Jersey and a French patrol vessel is heading towards the island, as fishermen protest over their post-Brexit rights.
About 60 French and Jersey boats are blocking the island’s St Helier port, with a freight vessel unable to leave.
French fishermen say their rights are unfairly restricted by licences issued by the island under a new system.
Jersey officials are meeting the “peaceful” protesters.
HMS Severn, which has previously been used to shadow Russian navy warships off the English coast, can be seen from the port, sitting off about a mile from the French boats. HMS Tamar is nearby and both ships are maintaining a presence and not making any effort to intervene.
No 10 said it sent the two Navy vessels to “monitor the situation”.
France has threatened to cut off electricity to the island over the row.
The boats are protesting against new fishing rules – introduced last week by the Jersey government under the UK-EU Trade and Cooperation Agreement (TCA) – which require French boats to show they have a history of fishing in Jersey’s waters. But it has been claimed additional requirements were added without notice.
French authorities say “new technical measures” had not been communicated to the EU, rendering them “null and void”.