A few weeks ago, a group of men knocked on my door. The three adults had large and unruly beards and wore shalwar kameez, the national dress of Pakistan. The two teenage boys accompanying them, in jeans and T-shirts, were in the initial stages of growing their beards. They wanted to know if I was regular with my prayers and if I knew how to recite the Koran correctly. Then they invited me to join them on their mission to "teach Muslims the correct way of performing their prayers".

The group belongs to Tablighi Jamaat, the mother of all grass-roots Islamic movements. It is now seen as the common link between several Muslims alleged to be involved in plans to blow up transatlantic airliners. Most British Muslims find it difficult to believe that the Tablighis, widely acknowledged as apolitical and harmless, could be implicated in such dastardly designs.

The Tablighi Jamaat was established in India in 1926 by Mawlana Muhammad Ilyas (1885-1944), a puritan, religious scholar. Devoted largely to the business of preaching, it has millions of followers in the Muslim world and the west. The Jamaat's annual conventions in Raiwind, Pakistan, and Tongi, Bangladesh, attract more than a million members from all over the world. At the meeting, everyone dresses in the same simple Pakistani dress and lives as austerely as possible.

The teachings of the organisation are based on "six points". Every Muslim must be able to make a correct declaration of faith, know how to perform ritual prayers correctly, inculcate a habit of remembering God, respect other Muslims, behave honestly and decently and should spend some time in passing on this message to other Muslims. These points form the core of Jamaat's textbook, Tablighi Nisab (Tablighi Curriculum), which is the only book the Tablighis ever read. Members are discouraged from asking questions, arguing, or making political comments. They are organised into mobile units and sent out to target Muslims lacking in faith. The object of the exercise is to lure the weak ones into the mosque, where they can be repeatedly subjected to the "six points" programme.

I became a Tablighi for a few days during my youth. Indeed, most young Muslims in Britain have spent some time "going out on Tabligh". It is difficult not to. The Tablighi are ubiquitous, do not give up easily and their simple message resonates with nascent minds. The secret of their success lies in direct, personal appeals and the emphasis on rituals. That is why they are most successful among the young.

But there are other reasons for the striking success of the Tablighis. They operate as a network, which changes constantly, and are not interested in developing permanent institutional structures. They shun all publicity and work quietly. The members are dedicated, highly motivated and spend their own resources for the work of the Jamaat. They have been encouraged by governments both in the Muslim world and the west, particularly as a counter balance to more politically active segments of the Islamic movements.

Conventionally, the Tablighis are seen as an unchanging, conservative, benign, global network of simple preachers. This, I think, is a serious mistake. Organisations do not remain static. Simply because Tablighi Jamaat has followed exactly the same course for decades, no one thinks it can change. It has. Drastically.

To begin with, there is not one but two Tablighi Jamaats. A breakaway group emerged in the mid-1990s and added a seventh point to the Jamaat's programme: jihad in Pakistan and abroad. In October 1995, a group of Tablighi soldiers from the Pakistani army were involved in a plot to overthrow Benazir Bhutto, the then prime minister. The plot was discovered; and Bhutto purged the army, sending a string of Tablighi officers into early retirement. But the new faction, for all intents and purposes quite indistinguishable from the old one, went on to establish its headquarters in the northern Punjab town of Taxila, from where it advocates active involvement in politics and jihad.

But even the old Jamaat is not what it used to be. It has been infiltrated by groups such as Lashkar-e-Toiba, the banned organisation responsible for sectarian violence in Pakistan. Office-holders in Lashkar-e-Toiba, and other militant organisations such as Harkat-ul-Ansar, openly boast that they recruit their volunteers from the Tablighi Jamaat. This doesn't surprise me. An unquestioning mind, which is what the Tablighi tends to produce, can easily be redirected towards nefarious ends.

So the Tablighis are not as harmless as most Muslims seem to think. The world has changed; and the Tablighi Jamaat has changed with it. The teenagers who knocked at my door may end up learning more than simply how to say their prayers correctly. We need to look at the Tablighis much more critically and see just what they are teaching our youth.