The warning was distributed to police chiefs nine days ago by the Yard’s anti-terrorist branch. It follows a series of operations by police and MI5 aimed at disrupting possible attacks by terrorists linked to Al-Qaeda.

The latest warning was sent to the chief constables of the 51 forces in Britain on December 2. It said there was evidence to suggest that Islamist terrorists were planning to carry out bomb attacks against “crowded areas” such as shopping centres, football grounds and train stations.

Senior officers say the alert is not routine and are to increase patrols and searches of potential suspects.

Whitehall officials say Britain remains on the second highest state of alert, Severe (specific). It was raised from the third highest state, Severe (general) to the highest, Critical, on July 7, the day of the London suicide bombings.

It was due to have been lowered on July 21, but the four failed attacks of that day delayed the decision. It was reduced to Severe (specific) in mid-August. Sir Ian Blair, the Metropolitan police commissioner, revealed two weeks ago that since then police had foiled two further attacks on Britain. Senior police officers have briefed the cabinet that the threat remained high.

Blair said police are preparing for attacks in the next few months. “The sky is dark,” he said. “Intelligence exists to suggest that other groups will attempt to attack Britain in the coming months.”

Terrorists linked to Abu Musab al-Zarqawi, head of Al-Qaeda in Iraq, are feared to have established a network in Britain. Some are involved in recruiting young Muslim men to travel to Iraq to fight the coalition. Others are believed to be planning suicide attacks in Britain itself.

Patrick Mercer, the Tory homeland security spokesman, said he was concerned about further attacks against the underground system: “The government needs to explain what further efforts it plans to make the tube safer.”

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