Britain threatened on Friday to open trade dispute proceedings against France if Paris imposes sanctions on London in a rapidly deteriorating standoff over post-Brexit fishing rights.
The warning, after France seized a British fishing boat this week, raised the stakes in a row that is part of a wider dispute over trading arrangements between Britain and the European Union, and could badly disrupt cross-Channel trade if it is not resolved.
British prime minister Boris Johnson will meet French president Emmanuel Macron briefly in Rome this weekend, offering a chance to ease tensions. An EU official said talks between the two and the European Commission would continue over the weekend.
But Brexit minister David Frost warned Commission Vice-President Maros Sefcovic on Friday that France will be in breach of a 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 be likely to 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.
Losing patience, 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.
Boat detained
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".
He suggested to BBC television that Britain could "increase the enforcement that we do on French vessels, to board more of them if that's what they're doing to our vessels".
France's ambassador to London was summoned to explain the country's actions on Friday. The exact timing and outcome of that meeting were unclear.
Johnson's spokesperson also said the prime minister and Macron were expected to meet on the margins of the G20 summit of the world's 20 biggest economies in Rome on Saturday and Sunday.
"He will discuss a range of issues," he added, without giving details, and said France remained a close and strong ally of Britain.
Paris did not immediately comment on his remarks.
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.
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.
