The Government has vowed to retaliate if France carries out a threat to block British fishermen from its ports over the post-Brexit fishing row, Downing Street said on Wednesday.
A government spokesman said the French ultimatum to disrupt trade and hamper energy supplies will be hit with "an appropriate and calibrated response".
In an escalation of the tensions, Paris warned that from Nov 2 it was ready to block British fishermen from its ports and that its customs officials could implement a "go slow" approach at its borders for incoming shipments from the UK.
A spokesman for Emmanuel Macron, the French President, said the country will "not let Britain wipe its feet on the Brexit deal" and said sanctions could be announced on Thursday.
Paris said a secondary range of measures could restrict energy supplies to Jersey, increasing the price, and implementing harsher customs controls on cross-Channel trade.
A UK government spokesman said: "France's threats are disappointing and disproportionate, and not what we would expect from a close ally and partner. The measures being threatened do not appear to be compatible with the Trade and Cooperation Agreement (TCA) and wider international law, and, if carried through, will be met with an appropriate and calibrated response. We will be relaying our concerns to the EU Commission and French government.
"The UK stands by its commitments in the TCA and has granted 98 per cent of licence applications from EU vessels to fish in our waters. All our decisions have been fully in line with this commitment. We also support Jersey and Guernsey's handling of fisheries licensing decisions, which has been entirely in line with the provisions of the TCA".
Mr Macron has failed in his attempt to convince fellow EU members to join the counteroffensive against Britain in the row over fishing rights.
A critical diplomat said: "If it's not an EU action, can the French actually do anything?
"They're in a state of ignorance, Emmanuel Macron's position is beleaguered."
A number of EU countries believe the French leader is using the row over fishing to attempt to secure more domestic support ahead of next April's presidential elections.
It was said France was considering "half-baked" retaliatory measures against Britain because it has failed to secure the backing of EU nations for a full-blown assault over fishing rights.
Paris is furious that just 15 permits have been granted to French fishermen to operate in Britain's coastal waters out of 47 applications.
European Commission and UK officials are currently locked in "boat-by-boat" negotiations over the excluded vessels.
A spokesman for the EU's Brussels-based executive said France had agreed to withdraw 17 of those requests because the vessels had supplied insufficient evidence that they are allowed to operate in Britain's waters after Brexit.
A further 15 licences are being debated, and could still be issued licences if further evidence is found for their right to fish off the UK's coast.
Gabriel Attal, an Elysee spokesman, warned France was drawing up a list of sanctions that it could make public on Thursday.
He argued that France was missing "almost 50 per cent of the licences that we have a right to" under the post-Brexit fishing pact agreed between the UK and EU in December last year.
The Elysee's war of words with Britain is an escalation of the fractious relationship between London and Paris since the EU divorce.
Clement Beaune, France's European affairs minister, said the measures could be ramped up over time.
"It's the first series of measures. Either this first series of measures leads to a dialogue about licences, then that's good," he told the French national assembly.
"Or these measures do not lead to the deal being implemented and we will take other measures, including on the supply of electricity for example."
The row is also threatening to spill out over to Jersey, because of the Crown dependency's refusal to accept French licence requests.
The UK has granted nearly 1,700 permits to EU boats to operate in its waters, meaning its 12-200 nautical mile zone.
However, most of the tensions surround Britain's six-12 mile zone, considered its most sacred fishing grounds.
British authorities have licensed some 100 French boats to fish in its territorial waters, while 75 have been rejected.
For Jersey, some 111 permanent licences have been issued and 31 temporary permits have been issued, while 75 have also been rejected.
Lord Frost, the Brexit minister, said on Wednesday night: "It is very disappointing that France has felt it necessary to make threats late this evening against the UK fishing industry and seemingly traders more broadly."
He went on to accuse the French of failing to formally communicate their plans with the British Government adding: "We will consider what further action is necessary in that light."