Britain said Saturday it has not ruled out invoking a Brexit dispute tool with the European Union for the first time, after France threatened trade reprisals in a row over fishing rights.

"No of course not, I don't rule that out," Boris Johnson told Sky News in Rome on the sidelines of a G20 summit, where he is due to hold a brief meeting with French President Emmanuel Macron.

"But what I think everybody wants to see is cooperation between the European allies and Emmanuel Macron," he added.

"And I share a common perspective (with them) which is that climate change is a disaster for humanity and that we have the tools to tackle it," he said, ahead of the UN COP26 summit that starts in Glasgow on Sunday.

France is incensed that Britain and the Channel Islands of Jersey and Guernsey have not issued some French boats licences to fish in their waters post-Brexit.

Britain's divorce from the European Union is governed by a pact called the Trade and Cooperation Agreement, which London and Paris each accuse the other of breaching.

The TCA allows for Britain or the EU to trigger a dispute settlement process, which is so far untested since Brexit took full effect at the start of this year.

France has warned that unless licences are approved it will ban UK boats from unloading their catches at French ports from next week and impose checks on all products brought to France from Britain.

Britain in turn warned on Friday that it may implement new checks on all EU fishing boats.

Macron said that Britain's "credibility" was on the line in the dispute, accusing London of ignoring the TCA agreed after years of tortuous negotiations.

"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," he told the Financial Times.

In his Sky interview, Johnson retorted: "If there is a (French) breach of the treaty or we think there is a breach of the treaty, then we will do what is necessary to protect British interests."

But in the buildup to COP26, he stressed that all sides had to keep their minds on the bigger picture.

The fishing dispute "is quite frankly small beer, trivial, by comparison with the threat to humanity that we face", Johnson said.