The UK is braced for France to impose extra customs checks and ban British fishing boats at some French ports from midnight as official sources said neither side was backing down in their post-Brexit row over fishing rights.

Boris Johnson’s spokesperson said Britain had “robust” contingency plans in place if France followed through on its threats to introduce more border checks or stop some British vessels reaching French ports.

France has been angered that some of its small boats are being denied permission to fish in the waters around the UK and Channel Islands. However, the UK insists its licensing regime is reasonable and it will continue to require boats to provide evidence that they have previously fished in those waters on four days over the last four years.

Johnson and the French president, Emmanuel Macron, bumped fists as they arrived at the Cop26 climate summit in Glasgow on Monday, but government sources said the two sides were still far apart in the row.

At a post-G20 press conference on Sunday, Macron said Britain must give ground or France would trigger threatened trade reprisals this week. “The ball is in Britain’s court. If the British make no movement, the measures of 2 November will have to be put in place,” he said.

The UK foreign secretary, Liz Truss, then escalated the tensions on Monday as she warned France it had 48 hours to back down on threats or Britain would begin dispute talks set out in the Brexit deal.

A spokesperson for Johnson suggested there would not be retaliatory UK trade measures but action could be triggered through the dispute mechanism. “Depending on if, or what, the French decide to do we will enact them as and when we need to,” he said.

A meeting to find a compromise, organised by the European Commission with officials from the UK, Jersey and France, was marked by a lack of movement by both sides on Monday, with sources describing the mood as sour.

The talks continued into the evening but EU sources said they expected the French government to “gradually” deliver on its threats from Tuesday to slow down movements of British lorries through their ports by imposing further controls and checks.

Jersey’s government was preparing to provide its fishers with financial support in the expectation that their vessels would not be able to land catches in French ports. The Jersey Fishermen’s Association (JFA) also called for the island’s authorities to respond in kind to the expected crisis by closing off the whelk and scallop fisheries to French vessels and banning dredging and trawling “with immediate effect for a period of six weeks”.

The Jersey government issued another 49 temporary licences until January to French boats on Monday, allowing time for new arrangements to be put in place.

French officials have said they will bar UK fishing boats from some ports and tighten customs checks on lorries entering the country unless more licences are granted for their small boats to fish in Britain’s waters.

The diplomatic row over fishing, a very small sector of the economy, threatened to overshadow the G20 talks, which took place in Rome at the weekend, and also so the Cop26 summit in Glasgow.

Almost 1,700 EU vessels have been licensed to fish in UK waters, equating to 98% of EU applications for fishing licences, the UK government says, but this figure is disputed in Paris.

Truss suggested on Monday that Macron may be making “unreasonable threats” because he has a difficult election looming.

Asked about whether France and the UK had come to an agreement, she told Sky News: “The deal hasn’t been done. The French have made completely unreasonable threats, including to the Channel Islands and to our fishing industry and they need to withdraw those threats.”

Truss said that if the French did not withdraw the threats, the UK government would use “the mechanisms of our trade agreement with the EU to take action”, which “could lead to taking direct action in trade”.

She said: “The French have behaved unfairly. It’s not within the terms of the trade deal. And if someone behaves unfairly in a trade deal you’re entitled to take action against them and seek some compensatory measures and that is what we will do if the French don’t back down.

“[The French must] stop threatening UK fishing vessels, stop threatening the Channel ports, and accept that we are entirely within our rights to allocate the fishing licences in line with the trade agreement, as we have done.”

Truss said she would “absolutely” take legal action in the coming days if France did not back down, adding: “This issue needs to be resolved in the next 48 hours.”

Asked why the row had emerged, she said: “You might say there’s a French election coming up.” Truss seemed angered by the dispute and said: “I’m not remotely happy about what has happened.”