The sharp growth in extremist websites is making recruitment much easier for al-Qaeda, according to Interpol head Ronald Noble.

"The threat is global, it is virtual and it is on our doorsteps," he said.

Mr Noble told a conference of police chiefs in Paris there were 12 sites in 1998 and 4,500 by 2006.

He said tackling radicalisation had been made far harder by the internet because many of the activities involved were not criminal.

Increasingly, he said, the individuals targeted were young and vulnerable and from middle-class backgrounds.

A researcher at the London-based International Centre for the Study of Radicalisation told the BBC that the number of radical websites was now far higher than the figure given by Interpol.

BDST: 1935 HRS, 23 September 2010