Special forces have been deployed to back British police after Paris attacks that killed 129 people, British newspapers reported on Sunday, as part of a wider boost in security measures.
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," Home Secretary Theresa 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 chaired a meeting of the government's emergency Cobra committee on Sunday to review Britain's security response the Paris attacks.
"The UK police and security services are working very closely with their counterparts in France and Belgium to identify all those involved and to pursue anyone who may have been involved in the preparation of these barbaric attacks," she told reporters. 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.