Intelligence agencies in the UK, southern Asia and the Middle East are detecting an increased volume of communications between suspected al-Qaeda cells as the organisation, led by the fugitive Saudi-born dissident Osama bin Laden, accelerates efforts to pull off a major operation in the days around the anniversary of the New York and Washington attacks.
According to sources in Pakistan, al-Qaeda activists there have been given a three-month deadline to work with associates from local hardline Islamist organisations to target Western interests in the region.
British intelligence sources told The Observer that the UK is third on the list of al-Qaeda's preferred targets - after the US and Israel, and confirmed last night that credible reports of planned al-Qaeda attacks had grown.
'The threat remains high and the background noise has been growing over recent weeks. It's a question of when, rather than if, they will attempt another spectacular,' said one Whitehall source.
Western security agencies predominantly rely on communications intercepts, which they correlate with tip-offs from informers and information from questioning arrested suspects to alert them to forthcoming attacks. Increases in the volume of communications between known al-Qaeda suspects also suggest that action may be imminent.
Abu Zubaydah, the top al-Qaeda operative arrested in March in Pakistan, has told his American interrogators that the terrorist group was planning a wide range of 'mass casualty attacks'.
Bin Laden is believed to be hiding in the rugged mountains that line the frontier between Afghanistan and Pakistan. Many of his followers remain in Pakistan.
Last week four al-Qaeda gunmen and three paramilitary policemen died in a shoot-out near Kohat, on the borders of the semi-autonomous tribal areas where an estimated 300 al-Qaeda fighters are hiding.
Investigators in Karachi, probing last month's bomb attack on the US consulate in the city, believe that al-Qaeda operatives are now linking up with local Islamic terror and crime groups to carry out their attacks.
The new warning is the second such alert issued by intelligence services in the last seven months. The CIA and MI6 issued a warning at the end of last year after interrogations of al-Qaeda suspects in Camp X-Ray in Cuba revealed that the al-Qaeda high command had regrouped after the bombing of Afghanistan and was already planning attacks across the world.
Moroccan intelligence sources told The Observer last month that they had been told by their American and British counterparts that al-Qaeda reconnaissance cells had been sent from Afghanistan after the bombing of the Tora Bora mountain HQ last December with detailed instructions to carry out attacks on Western and Jewish targets.
As a result the Moroccans were last month able to foil a 'spectacular' designed to hit British and American troops in the Strait of Gibraltar by arresting a cell of three Saudi al-Qaeda operatives. Following the destruction of their support infrastructure in Afghanistan last autumn, bin Laden's operatives have dispersed throughout the region and into the Middle East.