France is set to take punitive action on Tuesday as the UK refused to “roll over” in a dispute over post-Brexit licences to fish in British waters.
Downing Street said it had “robust” contingency plans in place if Emmanuel Macron’s government carries out threats to disrupt trade from midnight.
Foreign Secretary Liz Truss said the UK would take legal action under the UK-EU Brexit trade deal, while a tit-for-tat retaliation to French action has not been ruled out.
Ministers in Paris have warned they could block British boats from landing their catches in French ports and tighten customs checks in protest at what they claim is a refusal by the UK authorities to grant licences to French boats.
But the UK insisted it would only grant licences to boats which meet the criteria set out in the Brexit deal.
The deadline loomed as the skipper of a Macduff-registered trawler was still detained along with his boat and crew in Le Havre facing a fine of up to £63,000.
The un-named captain and his crew of three – including one man from Ayrshire – were held at the port on board the Cornelis Gert Jan last week.
Emmanuel Macron and Boris Johnson met briefly as the French president arrived in Glasgow for the Cop26 climate summit.
And officials were involved in talks convened by the European Commission in Brussels, although UK sources played down the prospect of any breakthrough.
The UK has granted licences to 98% of EU vessels which have requested permission to operate in British waters.
But the dispute centres on access for small boats, under 12-metres, wishing to fish in the UK six-to-12-nautical-mile zone.
The government in Paris was angry that the UK originally granted only 12 licences out of 47 bids for smaller vessels, a figure which has now risen to 18.
Only boats which can demonstrate they have fished in UK waters for one day in each of the years between 2012 and 2016 qualify for a licence.
The Elysee Palace said that without movement from the UK Government the retaliatory measures would come into force at midnight, the French news agency AFP reported.
A No 10 spokesman told reporters: “As you would expect, we have robust contingency plans in place. I’m not going to get into the detail of them here.
“It is the French that made these threats and we are continuing to call for them to step back from those threats.”
Ms Truss told BBC Radio 4’s Today programme: “Those threats are completely unwarranted... We will use the dispute resolution mechanism in the EU deal to take action.”
She added: “We are simply not going to roll over in the face of these threats.”
Ms Truss suggested Mr Macron’s hardline stance was motivated by domestic political concerns.
She told Sky News: “You might say there’s a French election coming up.”
The Government is considering other actions including “rigorous” enforcement processes and checks on EU fishing activity in UK territorial waters, measures it said would be consistent with the Trade and Co-operation Agreement.