Brussels rallied behind France in its dispute with Jersey on Thursday as the fishermen behind a seven-hour blockade of the Channel island threatened to return and begin a fresh "scallop war" with Britain.
Wading into the mounting political row, the European Commission accused Jersey of breaching the terms of Brexit trade deal and demanded the UK intervene to stop it "discriminating" against French vessels.
It added that under the Trade and Cooperation Agreement, signed last year, EU fishermen could not be subject to additional conditions and Jersey should have notified the bloc of any changes in advance.
However, the intervention was dismissed by Downing Street, which insisted on Thursday night that the Crown Dependency had the "right to regulate fisheries in their waters", and commanded the full backing of the Government.
In a statement, a spokesman said the UK would continue to work with Jersey to facilitate discussions with the commission, although sources pointed out that French ministers had repeatedly refused to engage with George Eustice, the Environment Secretary.
It followed a second round of talks between Boris Johnson and Jersey's Chief Minister earlier in the day, during which he reiterated his "unequivocal support".
While the two patrol vessels guarding the port of St Helier were due to return to port on Friday morning following the withdrawal of the flotilla, Downing Street added that it would "remain on standby to provide any further assistance Jersey requests".
Two French patrol vessels have also left the waters around Jersey, having been dispatched earlier in the day amid fears the blockade could turn ugly. It came after a number of French fishermen warned that further action was inevitable unless Jersey backed down over new limits on French vessels fishing in its waters.
On Thursday night it was reported that French fishermen were also threatening to blockade Calais in order to stop British goods from entering the EU, in a further bid to ratchet up pressure on the UK.
It is the latest in a series of warnings over the major French port, which is crucial to UK exporters to Europe and which sees between 7,500 and 8,500 lorries passing through on a daily basis.
In April more than 100 fishermen also attempted to stop hauliers carrying British fish from leaving the port of Boulogne-sur-Mer, Europe's largest seafood processing centre.
The fishermen also urged the French government, which has threatened to cut off electricity supplies to the island, to follow through with the threat.
"We will go back, and next time it will be war," one crewman nicknamed "Popeye" told The Telegraph while onboard the French vessel L'Omerta.
"It'll be just like in the Port-en-Bessin," he added, referring to one site of the scallop wars that have erupted intermittently between French and English fishermen since 2012.
Dimitri Rogoff, the president of the regional fisheries committee of Normandy, urged the French government to follow through. "The show of force is done. It is the politicians who must take over," he said. "Now, if we don’t get our way, we need the minister to turn off the lights."
The threat was issued after a meeting between a delegation of trawlermen and Jersey's environment minister failed to broker a solution to the row, which erupted when the island's authorities placed new restrictions on French vessels.
France has accused Jersey of introducing unilateral restrictions on where and French boats can operate and for how long, with a number of vessels also unable to secure access because they cannot prove their historic links to the fishing grounds.
On Tuesday Annick Girardin, the French minister of the sea, suggested Paris was prepared to cut electricity supplies unless Jersey backed down.
Her comments were followed by the announcement that French fishermen on the north coast would sail to St Helier in order to bring Jersey "to its knees", in turn prompting Mr Johnson to dispatch Royal Navy vessels on Wednesday evening.
In scenes likened to an invasion, a group of 60 boats, mainly large trawlers and dredgers, descended on St Helier around 6am on Thursday morning.
After setting off flares and blocking the entrance to the port, the protest remained largely peaceful, although at one point a French vessel appeared to ram an English boat, which was forced to return to shore. The vessels left shortly after 1pm once the meeting with Jersey’s officials had ended.
Jersey is now proposing a new forum to bring together fishing representatives to prevent a repeat of this week's clashes.
However, signalling that the row would continue, France’s European affairs minister Clement Beaune warned that Paris would not be "intimidated" by Britain's show of force.
"Our wish is not to have tensions, but to have a quick and full application of the deal," he told Agence France-Presse. "That's the case for Jersey and that's the case for the licences we are waiting for in the Hauts-de-France [region]."
Throwing its weight behind France’s claims, the European Commission said: "Any proposed management conditions have to be notified in advance to the other party, giving them sufficient time to assess and react to the proposed measures.
"The commission has clearly indicated to the UK that the provisions of the EU-UK TCA have not been respected. Until the UK authorities provide further justifications on the new conditions, these new conditions should not apply. The commission remains in close contact with France and the UK on the matter."
Under the terms of the trade agreement, the EU has the ability to seek redress through arbitration. However, the UK is confident that Jersey has complied with its requirements.
A government spokesman said on Thursday night: "The Trade and Co-operation Agreement brought in changes to fishing arrangements between the UK and the EU. Jersey authorities have a right to regulate fisheries in their waters under this agreement, and we support them in exercising those rights."
A senior source added: "If they have difficulty – as French fishermen seem to – rather than using threatening language or trying to blockade a foreign port, they should use the dispute resolution mechanisms in the TCA which we’ve literally just ratified."
Speaking to BBC Radio 4's World at One, Ian Gorst, Jersey's external affairs minister, said: "It really is important that what has escalated over the course of the last three days, that we move away from that escalation and disproportionate threats, and start to deal with those detailed technical issues.
"It's become apparent over the course of the last few days, that perhaps some of the data and evidence that needed to be supplied to allow for licences to be issued to the full nature and extent of fishing vessels.
"But let's remind ourselves, we gave a four month amnesty in order to supply that information, and we've given a further two months amnesty in order that small boats, who would find it more difficult to give that evidence, can do so."