1:30pm UK, Monday August 01, 2005

Police are still on high alert in London amid fears that a third terrorist cell could be plotting another strike on the capital.

Thousands of officers are on the streets and guarding Tube and overland rail stations as the huge security operation continues.

180 Armed police patrol london security

Armed police patrol streets of London

The latest fears come despite all four suspected would-be bombers from July 21 being in custody.

Security sources have reportedly told The Times newspaper that a third group, believed to be made up of British-born Muslims, was on the verge of staging an attack on the Tube system last week.

The men are thought to be of Pakistani origin, and have links with the Leeds-based terrorist cell which carried out the July 7 bombings which killed more than 50 people.

However, a spokeswoman for the Metropolitan Police said the possible existence of a third team of bombers was "media speculation".

"We can't guarantee that there won't be a third attack, but we have never talked about a third cell."

She added: "We are remaining vigilant and operations continue."

British Transport Police have been targeting specific ethnic groups for "intelligence-led" stop-and-searches as part of their heightened security measures.

BTP Chief Constable Ian Johnston said his officers would not "waste time searching old white ladies".

Meanwhile, Italian authorities are considering handing back one of the four July 21 bombing suspects as UK police continue to question a growing number of people arrested in connection with the attacks.