France and other EU nations are racing to isolate cases of the highly infectious variant of Covid-19 dominant in parts of England while preparing for a possible tightening of restrictions on movement if the mutation threatens to spread widely.

Jean Castex, French prime minister, told parliamentarians at a meeting on Monday that it was impossible to rule out a third lockdown for the country, according to officials.

Arnaud Fontanet, an epidemiologist and member of the government’s scientific council on the pandemic, said it was important to consider closing the border to countries such as the UK in the face of the “very serious threat” posed by the new variant.

“By midweek we’ll know the size of the enemy and we’ll have to take appropriate measures,” Mr Fontanet told BFMTV. Jean-François Delfraissy, who heads the council, said last week that France probably “cannot avoid” the rapid spread of the variant, which took two and a half months to reach 60 per cent of new viral infections in parts of the UK.

Philippe Froguel, a professor of genomics at Imperial College London and at Lille in France, said very little genetic sequencing of the virus had so far been done in France compared to the UK, and that it was likely the new variant was already well established.

By February, he said, “I’m afraid we’ll be in exactly the same situation as the UK with 50-60,000 new cases a day and a large number of dead.”

French health workers have already found more than two dozen cases of the English Covid variant in Marseille in the south, Lille in the north and in the Alps in the south-east, but fear there may be hundreds or thousands more undetected infections. At least seven people were found to be infected with the English variant from a cluster in Marseille originating in a family of five who had returned from their base in the UK for the Christmas holidays.

Benoît Payan, mayor of Marseille, said on Sunday that there was a “worrying” reality of virus infections in the city and “every minute counts in the effort to control the English variant”. Marseille is in one of 23 French departments now operating with an extended night-time curfew between 6pm and 6am. For the rest of the country, including Paris, the curfew begins at 8pm.

A similar pattern is visible in other parts of western Europe, even as governments struggle to implement mass vaccination programmes that initially target health workers, the elderly and the vulnerable for protection.

Ireland detected the UK variant for the first time only on Christmas Day, but it has since had a big impact on surging infections in the country, amplifying coronavirus transmission after social restrictions were eased in the run-up to the holiday.

Rising infections have alarmed health officials and placed huge strain on the country’s hospitals, with the 14-day coronavirus incidence rate per 100,000 population at 1,291 on January 9, up from up from 166 on December 23.

“The UK variant has had a very significant impact in terms of the rapidity of the growth of transmission, of that I am convinced,” Micheál Martin, the prime minister, told NewsTalk radio on Monday.

Although the taoiseach said “socialisation” in December was also a factor in the surge, there was evidence that the UK variant was “growing in terms of its percentage of the overall transmission”. Official data suggest the variant accounted for 45 per cent of 92 samples sent for additional testing, up from 25 per cent in the week to January 3.

Chart showing that UK’s Covid hospital occupancy is now among the highest in the world

In Belgium, infection rates in Brussels have leapt by more than three-quarters in a week, in what some observers fear may be the impact of people returning to the European institutional hub after travelling over Christmas and the new year.

The Belgian capital, which is the headquarters of both the EU and Nato, saw a 76 per cent rise for the seven-day period to last Thursday compared with the previous week, according to official figures. That has redoubled concerns that more cases of the UK variant will appear, adding to the handful already discovered in Belgium.

Denmark has toughened travel restrictions in response to the spread of the British and South African coronavirus strains, barring entry since the weekend to all international air arrivals unless they have a negative Covid-19 test from the 24 hours before they boarded the aircraft.

The European Commission declined to comment specifically on the new Danish travel rules, but said it was “closely monitoring” the situation. It added that it had been “pushing for increased and better co-ordination” at an EU level on border controls.

Spain said last week that it had detected at least 60 cases of the English strain but added that the total could increase significantly because of the number of cases being studied.

Information about the prevalence of the new strain reflects not just the infection rate but also countries’ differing abilities to detect it. Spain has had problems compiling national data since the onset of the pandemic, partly because of differing methodologies among the country’s 17 regions, which have primary responsibility for health policy.

Last week a Spanish genetics laboratory in Valencia detected the new strain in two patients with no known links to the UK, indicating community transmission had begun. The samples had been taken two weeks before. France has also found at least two cases with no direct link to the UK.

“It’s clear that there is community transmission, but we don’t know at what level,” said Fernando González Candelas, a researcher at Valencia's Fisabio foundation and professor of genetics at the university of Valencia. Noting that the Ramón y Cajal Hospital in Madrid had found the new variant in only 1 per cent of the samples it tested for it, he added: “There are indications that it is low, but we can’t be sure of that.”

Additional reporting by Daniel Dombey in Madrid and Arthur Beesley in Dublin