Jayne Adye was commenting after the detention of a UK trawler in a French port in an escalation of a diplomatic row triggered by French claims over a perceived shortage of licences for French boats to fish in UK waters. French ministers have said they will block British boats from some French ports and tighten checks on vessels travelling between France and the UK if the issue is not resolved by Tuesday - as well as threatening the electricity supply to the Channel Islands.

       Ms Adye, director of grassroots, cross-Party campaign Get Britain Out, told Express.co.uk: "It is completely unacceptable for French President Emmanuel Macron to try and blackmail the UK Government in the row over fishing licences, by condoning the arrest of a UK trawler outside Le Havre.

       “Macron seems determined to try and placate the militant French fishermen in advance of the French Presidential race next year.

       “Clearly, he is trying to capture their votes, as currently, one of the serious contenders in the Presidential race is Marion Anne Perrine "Marine" Le Pen, who many French fishermen currently support.”

       The ongoing row over fishing licences was not going away, with figures on the exact number of licences issued on both sides of the Channel being disputed by each, Ms Adye stressed.

       There were specific rules which have to be adhered to in order to prove who has fished in UK waters previously, and for which fish, in order for a licence to be issued.

       French boats not issued with licences apparently have not yet provided adequate information, she explained.

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       Ms Adye said: “However, this escalation overnight, following meetings between France and Jersey yesterday (and Jersey releasing a large number of new licences – clearly not enough, as far as the French are concerned), seems to be far more about French domestic politics and President Macron trying to blackmail the UK Government into granting unlimited numbers of licences, and the very real prospect of overfishing in UK waters.

       “It seems completely irrational for the French to somehow believe they have a unilateral right to fish in UK waters and effectively demand an unlimited number of licences, willy nilly, and if they can’t get their own way they will retaliate by threatening to blockade ports and/or seize UK fishing vessels in the way they have done today.”

       Anger is rising in London at the situation, with Britain telling France "two can play at that game" in a warning the UK could retaliate if Paris goes ahead with "inflammatory" threats made in a post-Brexit row over fishing rights.

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       Foreign Secretary Liz Truss will challenge Paris's ambassador to the UK Catherine Colonna on France's intentions on Friday after taking the rare step of ordering an allied nation's envoy to be summoned.

       Environment Secretary George Eustice did not rule out blocking French vessels in return as he struck out at a claim from France's Europe minister Clement Beaune that the only language Britain understands is "the language of force".

       Mr Eustice told BBC Breakfast: "That is completely inflammatory and is the wrong way to go about things."

       Asked how the UK will respond if France does go ahead and block British trawlers, the Cabinet minister said: "Two can play at that game."

       He insisted any British response would be "proportionate", adding: "It's always open to us to increase the enforcement we do on French vessels, to board more of them if that's what they're doing to our vessels - there are other administrative things we can require of vessels."

       Asked whether the Government could block French vessels landing their catches in the UK, he responded: "If the French obviously do continue with this, then yes, we will take a proportionate response to that."

       Mr Eustice acknowledged that the France detaining the British vessel may have been a "routine operation" but has received mainstream attention because French authorities last week "said they were going to introduce all sorts of problems and make life difficult for people".

       The scallop vessel Cornelis Gert Jan was caught up in the diplomatic storm, and ordered to divert to the port of Le Havre on Wednesday after the French authorities said it was fishing in French waters without a licence.

       French officials said that another British trawler had also been fined for obstruction after refusing to allow police to board to carry out checks.

       The owner of the Cornelis, Macduff Shellfish, said the vessel had been fishing legally in French waters and called on the British Government to protect the rights of British fishermen.