As British anti-terror police interrogated 11 Pakistanis on Friday over what Prime Minister Gordon Brown called a "major terrorist plot", security sources have indicated that at least three quarters of terrorist plots under investigation in the UK have their roots in Pakistan.

While Afghanistan was seen as the training ground of terrorists in the aftermath of the September 2001 attacks, recent experience has shown that an increasing number of al-Qaeda extremists are being trained across the border in the tribal areas in north west of Pakistan.

The alleged plot to bomb shopping centres in Manchester during this Easter holiday has been linked by MI5, Britain's intelligence agency, to two al-Qaeda suspects in Pakistan - Briton Rashid Rauf and Baitullah Mehsud, the leader of Pakistan's Taliban movement.

An estimated 4,000 British Muslims have been trained in terrorist camps in Pakistan, and with 400,000 British citizens visiting Pakistan each year, there are fears many more will become radicalised, The Daily Telegraph reported on Friday.

Monitoring of visitors has intensified, which has raised the possibility of a change in tactics by terrorists, using Pakistani nationals who may not be so closely watched when they visit Britain.

According to the report al-Qaeda was days away from carrying out an "Easter spectacular" of co-ordinated suicide bomb attacks on shopping centres in Manchester.

Police rounded up 12 people, eleven of them Pakistanis here on student visa, and some of them were watched by MI5, the British intelligence agency, as they filmed themselves outside the Trafford Centre on the edge of Manchester, the Arndale Centre in the city centre and the nearby St Ann's Square.

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