ROME/LE HAVRE, France, Oct 29 (Reuters) - French President Emmanuel Macron on Friday escalated a post-Brexit row raging between Paris and London when he questioned Britain"s "credibility" and accused it of wanting to change parts of a treaty signed just last year over fishing rights and other topics. Speaking to the Financial Times, Macron said other nations were watching closely as Britain clashed with France and the broader European Union following its departure from the bloc. Britain, looking to strike new trade deals after leaving the EU, had threatened to open trade dispute proceedings against France if Paris imposed sanctions on London. Such a step could seriously disrupt trade between the neighbours. "Make no mistake, it is not just for the Europeans but all of their partners," Macron said. "Because when you spend years negotiating a treaty and then a few months later you do the opposite of what was decided on the aspects that suit you the least, it is not a big sign of your credibility." The comments come before Macron and British Prime Minister Boris Johnson are due to meet briefly this weekend at the G20 meeting in Rome, offering a chance to ease tensions. On the flight from London, Johnson told reporters France was "one of our best, oldest, closest allies, friends and partners", and that he would make that clear to Macron when they met. Johnson added that he was puzzled by the fishing dispute and feared Paris may have breached terms of the Brexit trade deal. "Obviously we’re going to take the appropriate action," he added. Britain"s departure from the EU last year deepened strains in its relations with France, and negotiations on fishing rights proved long and difficult even before it left the bloc. France says Britain has refused to grant its fishermen the full number of licences to operate in British waters that France says is warranted. Britain says it is issuing licences to vessels that can prove they have previously fished in UK waters. Tensions rose sharply when a British scallop dredger, the Cornelis Gert Jan, was escorted to the northern French port of Le Havre overnight on Wednesday after French officials said its crew failed to prove it was allowed to fish in French waters. British officials said it had the correct documentation. Angered by the incident, and by the threat of sanctions, British Environment Secretary George Eustice said on Friday: "Two can play at that game". Britain"s Environment Secretary George Eustice walks outside the Science Museum, following a cabinet meeting in London, Britain, October 19, 2021. REUTERS/Toby Melville A British trawler Cornelis Gert Jan is seen moored in the port of Le Havre, after France seized on Thursday a British trawler fishing in its territorial waters without a licence, in Le Havre, France, October 29, 2021. REUTERS/Sarah Meyssonnier Britain"s Environment Secretary George Eustice walks outside the Science Museum, following a cabinet meeting in London, Britain, October 19, 2021. REUTERS/Toby Melville 1/2 A British trawler Cornelis Gert Jan is seen moored in the port of Le Havre, after France seized on Thursday a British trawler fishing in its territorial waters without a licence, in Le Havre, France, October 29, 2021. REUTERS/Sarah Meyssonnier THREAT AND COUNTER THREAT Brexit minister David Frost also warned Commission Vice-President Maros Sefcovic on Friday that France would be in breach of the post-Brexit free trade deal if Paris carries out its threat to impose sanctions on Britain from Tuesday. "Lord Frost made clear that, if these actions were implemented as planned on 2 November, they would put the European Union in breach of the Trade and Cooperation Agreement (TCA)," a British government spokesperson said. "The government is accordingly considering the possibility, in those circumstances, of launching dispute settlement proceedings under the TCA, and of other practical responses." Such proceedings would likely involve convening an arbitration panel to decide on the dispute, and could result in a demand for compensation or suspension of obligations under the free trade deal. Paris has said it will ban British fishing boats from unloading in French ports, carry out additional licence checks on British vessels, tighten controls of trucks and reinforce customs and hygiene controls if talks fail. It has also hinted at the possibility of later raising energy tariffs to Britain. Britain has responded by summoning France"s ambassador to London to explain the country"s actions. Environment minister Eustice said Britain could also "increase the enforcement that we do on French vessels, to board more of them if that"s what they"re doing to our vessels". Some British officials portray France"s defence of its fishermen as an attempt by Macron to show he is looking after their interests before an election in April in which he is expected to seek a new term. Johnson can also ill afford to look weak on fishing rights after leading the campaign to leave the EU and promising that leaving the bloc was in voters" interests. Fishing makes a small contribution to the French and British economies but is a lifeline for some coastal communities.