France is preparing to implement a go-slow strategy for customs checks on shipments to and from Britain ahead of Christmas in an escalation of the post-Brexit war over fishing rights, officials fear.
Paris will on Tuesday rubber-stamp a package of planned retaliatory measures that could be triggered if French fishermen are not granted greater access to Britain's coastal waters.
The Telegraph understands the prospect of French customs officials deliberately disputing cross-Channel trade is a chief concern amongst British officials.
French fishing leaders are also threatening to blockade shipments in and out of Calais from Saturday morning unless they are granted more permits to operate off Britain's coast.
Jean Castex, France's prime minister, will warn EU and UK negotiators - who on Monday continued high-stakes discussions over post-Brexit fishing licences - that they have until midnight on Friday to resolve the row.
France is furious that the UK has approved just 15 permits for small French fishing boats to operate in the zone between six to 12 miles off its coast, out of 47 applications
The row threatens to plunge Franco-British relations to a new low. Paris and London have also been at odds over Chanel migrant crossings and the UK's Aukus nuclear submarine with Australia and the US, which cut out the French.
Responding to mounting pressure from French trawlermen, Mr Castex will on Tuesday unveil a package of planned reprisals that could be triggered as early as November 1.
It will include cutting energy supplies to the UK and Jersey and blocking Britain's fishing fleet from entering French ports.
He is unlikely to secure support for hitting British exports with trade tariffs from the European Commission or EU capitals, some of whom believe Mr Macron is using the row as part of his re-election campaign ahead of next year's presidential ballot.
But Paris could order its customs officials to cause backlogs by carrying out more physical checks on shipments between Britain and France, and vice-versa.
Companies could also be slapped with new surcharges for the more vigorous controls on the French border.
Annick Girardin, France's maritime minister, has reassured French fishermen that she "won't give up" until they're granted more licences to fish off the coasts of Britain and Jersey.
"We are one week away from the deadline set by France for answers from the UK," she said. "I want to give clear visibility to the fishermen and remind them the French government is by their side."
"I want my licences back," she added, in reference to Margret Thatcher's renowned outburst during budget talks at an EU summit in 1979.
French officials are more cautious over the prospect of retaliatory measures. One said: "We'll have to stay within proportionate sanctions so as not to further affect the already well-deteriorated relationship with the UK."
Raising the prospect of militant action by the country's trawlermen, France's powerful National Fisheries Committee criticised the slow progress being made during the EU and UK's "vessel-by-vessel" negotiations over fishing licences.
"The negotiations that resumed last Wednesday have led to the issuance of a handful of additional licenses at this stage," the organisation said. "The progress made is far too timid and especially far too long."
Britain insists it is working constructively to help French fishermen prove their status to operate in UK waters under the Brexit trade agreement.
A government spokesman said: "We're continuing to work closely with the European Commission and French government and we remain open to considering further evidence that supports the remaining licence applications.
"Defra have recently granted a further three licenses to French vessels under 12 meters as a result of new evidence supplied by the Commission."