Michel Barnier has accused the Government of behaving like "buccaneers" and failing to respect the Brexit fishing deal following the Jersey stand-off.
Mr Barnier, the EU's former chief Brexit negotiator, waded into the debate, saying: "I think that the British are behaving like buccaneers – and it's not the first time."
On Thursday, around 60 French vessels attempted to blockade St Helier, Jersey's main port, in protest at the licences.
Local leaders in La Manche, Normandy, said boats from the Channel island would be suspended from entering the ports of Granville, Barneville-Carteret and Dielette until further notice.
Speaking on French political TV programme C Politique on Sunday, Mr Barnier added: "I think the French fishermen are right and they must be supported.
"The French authorities, the government and the European Commission must support these fishermen because they have the right to obtain the right to fish in these waters near Jersey and Guernsey, where there are less problems."
He said the post-Brexit trade deal is "not currently being respected", adding: "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.
"I think that the people of Jersey should be very careful as well because they depend on the [continent] not just for electricity but for business activity and VAT. We have a global deal and they must be careful, that's the message I want to pass on."
On Friday, Ian Gorst, Jersey’s minister for external relations, said the island regretted the decision by French ports to ban Jersey fishermen from landing their catch and believed it fell foul of the Brexit trade deal.
"For that reason, we are referring the notice of this decision immediately to the European Commission," Mr Gorst said in a statement.