The prime minister ordered Royal Navy vessels to the British dependency last week after a fleet of 60 French vessels tried to blockade Jersey’s main port in protest over restricted licences.
Mr Barnier accused the UK government of failing to respect the Brexit agreement on fishing access – saying the French fisherman were “right” to feel aggrieved.
“I think that the British are behaving like buccaneers and it’s not the first time,” the Brexit trade deal negotiator told French TV.
“I think the French fishermen are right and they must be supported,” Mr Barnier said. “The French authorities, the government and the European Commission must support these fishermen.”
The French politician added: “They have the right to obtain the right to fish in these waters near Jersey and Guernsey.”
French seamen in Normandy have accused the Jersey authorities of failing to issue new, legal licences needed to fish in its waters.
The French government has backed its domestic industry in the dispute, with one of Emmanuel Macron’s ministers threatening to cut off Jersey’s electricity supply.
Maritime minister Annick Girdardin suggested the move during a speech in the French parliament, warning Paris was ready to use “retaliatory measures” outlined in the Brexit trade agreement.
Mr Barnier suggested the threats to the electricity were credible. “We must clearly tell the British it cannot work like this otherwise there will be serious consequences on the deal in general and reprisal measures that are included in the treaty,” he told the France 5 network.
“I think that the people of Jersey should be very careful as well because they depend on the [EU] not just for electricity but for business activity and VAT,” Mr Barnier added. “We have a global deal and they must be careful, that’s the message I want to pass on.”
With the issue unresolved, local fishing industry leaders in Normandy have said any boats from the Channel Islands would be blocked from entering their ports.
Cabinet Office minister Michael Gove said on Monday that UK officials – including the environment secretary and Brexit minister were currently trying to resolve the matter with their French counterparts.
“I know George Eustice and Lord Frost have been having conversations with French and EU counterparts to deescalate the situation,” he said.
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