Extra police officers were being deployed at transport hubs in London on Friday as the British capital was put on alert amid fears of a terrorist attack, reports said.

Transport police have cancelled leave and called in extra officers and there will be a heavy presence across the capital and at Heathrow airport, Sky News television and The Daily Telegraph newspaper reported.

A security source told the Telegraph that there was no "imminent" threat but there was activity from one of a handful of extremist cells that cause concern.

"Intelligence chatter" had reportedly suggested that transport hubs in London could be a target and this had led to an adjustment in policing levels.

The overall threat level to Britain had not changed and remained at the second-highest level of "severe" on Friday, meaning an attack is considered highly likely, said police and the government

London's Metropolitan Police refused to comment on the reports but said: "The threat level to the UK is at severe, which means that an attack is highly likely, and has been since January 2010.

"We will police accordingly and use a range of covert and overt tactics which remain under constant review."

A spokesman for the Home Office, or interior ministry, added: "There is a continuing need for everyone to remain vigilant and to report any suspicious activity to the police."

The threat level of "severe" is the second highest on the five-stage grading with the top level being "critical", meaning an incident is expected imminently.

The threat level was hiked in January last year after a six-month spell at "substantial" -- the only time it has dipped below the two highest levels since it was set up in 2006.

The system was established following the 2005 London bombings, in which 52 passengers were killed in suicide attacks on three trains and a bus.

The reports of heightened security came after a December 11 suicide bombing in Stockholm was linked to Britain, with the bomber having lived in Luton, just north of London, for the past few years.

It also came after nine men were charged at the end of last month with planning attacks, reportedly in a pre-Christmas plot with a string of high-profile targets including the London Stock Exchange and the US embassy.