Interior minister Theresa May did not deny the reports, telling the BBC that “arrangements” had been made to give the police military support where necessary.

Unsourced British media reports said special forces were dressed in plain clothes and supporting police at busy public locations, including train and metro stations, shopping centres and popular entertainment districts.

“There are tried and tested arrangements in place to give military support,” May told the BBC when asked about the veracity of the reports.

“I don’t comment on the particulars of any deployments that are made, but we have arrangements in place where necessary for the police to have military support.”

On Saturday, Mark Rowley, the National Police Chiefs’ Council lead for counter-terrorism, said that policing at ports had been strengthened.

“People may notice some changes at events at big cities across the country,” he added.

Britain’s “severe” threat level, in place since August last year, remained unchanged after the Paris killings.

The threat level means an attack is “highly likely”, and the next highest level, “critical”, means an attack is imminent.

May is expected to chair a meeting of the government’s emergency Cobra committee later on Sunday to review Britain’s security response the Paris attacks.

One Briton was confirmed killed in the co-ordinated assault on the French capital, and the government said a “handful” of others were also feared dead.