LONDON, Sept. 21 (Xinhuanet) -- The likelihood of another terrorist attack on the UK, and London in particular, continue to be high, with police being forced to change their tactics constantly to disrupt or thwart any terrorist attempt, a senior police official has said.

    Peter Clarke, head of Scotland Yard's anti-terrorist branch, was quoted by Wednesday's Financial Times as saying on Tuesday that there is no firm information suggesting that any particular building or event has been singled out for attack.

    He said that police hope their tactics will at the very least prevent any potential terrorist identifying a vulnerable target.

    Thousands of police who were deployed to central London for counter-terrorist operations in July have returned to crime-fighting duties in their local units, and to forces outside the capital, according to Whitehall sources.

    But Clarke emphasized that the public should not interpret thisas a sign that police were lowering their guard. "What we have is a flexible and constant changing deployment," he said. The tacticsare thought to include police patrols on London Underground trainsand ongoing covert operations in addition to continuing deployment of armed units.

    Clarke said the bombings on July 7, which killed 52 people, were likely to have been planned "over months" with the terrorists carefully choosing the timing and location for their attacks on the London transport system.

    The latest CCTV footage released by police investigating the attacks on July 7, shows that three of the four terrorists who were directly involved in exploding the bombs, rehearsed their operation nine days earlier, in an apparent attempt to test security and schedules.

    The footage shows Mohammad Sidique Khan, Shehzad Tanweer and Germain Lindsay, two of them carrying rucksacks, meeting at Luton railway station (35 miles north of London) on the morning of June 28 and then arriving at King's Cross 50 minutes later. All three terrorists were killed when their bombs exploded on the London Underground. Enditem