A picture has emerged from the G20 summit in Rome of British Prime Minister Boris Johnson and French President Emmanuel Macron smiling at each other and knocking elbows during a photo opportunity. Between them stands European Commission President Ursula Von Der Leyen.

Could she be the peace broker in this escalating dispute between arguably the EU's most influential head of state and the leader of the country that has just left the bloc?

The words coming out of both leaders' mouths at the moment suggest that is looking increasingly unlikely.

Johnson met with Von der Leyen on Saturday. According to Reuters, Johnson described France's seizing of a British fishing vessel and threats to step up checks on UK exports "completely unjustified."

Johnson also repeated a claim he has made several times, that the British government believes France may be breaking international law with such moves. 

"We are a bit worried France might be about to come into breach – or may already be in breach – of the trade and co-operation agreement that we struck," he told Sky News.

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France is acting unilaterally in this dispute so far. The European Commission officially represents Paris on fishing policy but other EU member states are yet to explicitly back President Macron's position.

Nonetheless, the UK's Brexit negotiator Lord Frost appears keen on painting Brussels as part of the problem.

Frost has reportedly called on "France and the EU to step back from rhetoric and actions that make resolving differences more difficult."

According to Reuters, Frost has said the British government is actively considering triggering a dispute mechanism built into the Brexit free trade agreement to deal specifically with fisheries disputes.

That could get messy. An arbitration panel could be convened to consider both sides' cases, demands for compensation could be made and in a worst case scenario, the whole hard-fought EU-UK free trade deal could be suspended.

A trade war over whether a few dozen French fishing boats should be allowed to continue to fish around the small British Crown Dependency of Jersey - just off the French coast - might seem over-blown. But Paris has already suggested it might be prepared to step up its measures to persuade Britain to concede and grant more French fishing licenses, for example by reducing the supply of electricity from France.

That would hit a sore spot amid a record surge in energy prices that would surely illicit a similarly firm response.

The French side are insistent, though, that Britain is intentionally making life difficult for French fishing fleets by not renewing licenses.

In an interview with the Financial Times this weekend, President Macron said Britain's credibility was at stake, not just on upholding the terms of the fisheries clauses in the Brexit deal but also on those that deal with the Northern Ireland Protocol which British officials now say are unworkable and want re-written. 

Such an accusation from a close partner at a time when Britain is seeking to negotiate and sign trade deals with other non-EU countries after Brexit will hurt. But at the moment, both sides appear to believe that the pain of backing down - and the post-Brexit precedent that might set - remains the greater threat.