Then the teacher let me peer through the door to see hundreds of boys in white pyjamas and prayer caps sat on the floor hunched over large books and rocking back and forth. The sound was their muttering as they tried to memorise all 77,934 words of the Koran.

This was Darul Uloom Haqqania or House of Knowledge, one of Pakistan’s leading madrasahs based in Akora Khattak in the North West Frontier Province. The Eton of budding Islamic warriors, its 2,500 places are heavily oversubscribed. Upstairs in the hall leading to the Library of Fatwas, a roll of honour lists most of the Taliban leadership as alumni as well as an honorary degree for Mullah Omar.

Madrasah is the Arabic word for religious school and the only lessons were Arabic, Islamic jurisprudence and learning the hadith, the sayings of the Prophet. Students are also taught the proper size for a beard and appropriate trouser length. There is no science, maths, literature or other languages and everything was by rote learning.

“Why do we need discussion?" asked my guide Rashid, the deputy director, when I questioned this. “What is written is written.”

For one used to a western lifestyle, the students — aged from five to their twenties — seemed to inhabit an almost prison-like life. Up at 4am for the first of five prayers a day, they sleep on thin mats on the floor in unheated dormitories.

Greying washed shirts hung stiff on a line outside. The only posters on the walls were of a Kalashnikov-wielding Osama Bin Laden on a charging white horse. A large boom box stood on the floor but the tapes alongside were of sermons from radical imams.

The teenagers I spoke to were unable to do simple calculations and had never heard of dinosaurs. They laughed uproariously at the idea that man could walk on the moon.

When I asked what they wanted to be when they graduated, they talked of becoming mullahs. One or two spoke of embracing shahadat, martyrdom, and of going to paradise with its 72 virgins, almost as though this world was just a grade to get through.

My visit was short — as a woman, although clad in an all-encompassing burqa, I had been warned I might be stoned and my questions were clearly provoking some hostility.

But I have known the director Maulana Sami-ul Haq since I lived in Pakistan in the late 1980s and he waved me to a plastic chair in his car port and offered Pepsi, brought by his son Osama. The maulana explained that his father had founded the school before independence in 1947, stating that “we don’t have money or guns to drive out the British but through education we have the power to influence and raise an entire generation against them”.

The only foreign students I had seen were central Asian, Indonesian and north African, but the maulana boasted that among the thousands of applicants every year were British students.

It was in one such madrasah near the old Mughal city of Lahore that Shahzad Tanweer, the Aldgate bomber, spent time at the beginning of this year after growing up in Leeds. It was a trip from which friends say the cricket-mad 22-year old returned a changed man.

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