ROME (Reuters) -British Prime Minister Boris Johnson said on Saturday he could not rule out triggering trade dispute action against France next week in a post-Brexit row over fishing that has further strained relations and could ultimately disrupt the flow of goods.

       Johnson, who is hosting the U.N. climate summit next week, again said he did not want the spat over fish to derail a meeting of the world's 20 biggest economies, seen as a stepping stone to secure more commitments for COP26 in Glasgow https://www.reuters.com/business/cop.

       After watching an earlier British-hosted G7 meeting becoming overshadowed by a disagreement with the European Union over post-Brexit problems with the movement of sausages and other goods to Northern Ireland, Johnson was keen to defuse the row.

       Relations with France have become increasingly strained since Britain voted to leave the EU in 2016, with London's security pact with the United States and Australia doing little to build trust with Paris. French President Emmanuel Macron has questioned Britain's "credibility".

       Fishing, which dogged Brexit talks for years, while not economically crucial to either country, holds huge political importance to both, and the row, if not resolved, could trigger the beginning of dispute measures in the Brexit trade deal.

       "If there is a 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," Johnson told Sky News from Rome, where he is attending the meeting of G20 leaders.

       Asked if he would rule out triggering dispute resolution measures in the so-called Trade and Cooperation Agreement (TCA) next week, Johnson said: "No of course not, I don't rule that out."

       "But what I think everybody wants to see (is) cooperation between the European allies and Emmanuel Macron and I share a common perspective which is that climate change is a disaster for humanity."

       'CALMING THINGS DOWN'

       Any TCA 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.

       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.

       The issue escalated this week when a British scallop dredger was escorted to a French port after French officials said it did not have the correct documentation.

       Both sides have since threatened to take further action, but a French diplomatic source told Reuters on Saturday that Macron shared Johnson's aim of easing tensions.

       "The president is in favour of calming things down, but at the same time he can't pretend the British are not reneging on the commitments they've made," the source said.

       "Brexit was a sovereign choice, we respect that. It's taken years to negotiate. The deal was signed, it must be applied now. When you sign a piece of paper, you must be true to it."

       With an election in April in which Macron is expected to seek a new term, some British officials believe the French president is seeking to look tough over fishing rights to appeal to his electorate.

       Meanwhile, some Europeans diplomats see Johnson's government as likewise taking a firm stance to please Brexit supporters.

       (Reporting by Elizabeth Piper, Michael Holden in London and Michel Rose in ParisEditing by Alison Williams and Helen Popper)