The Home Secretary is to outline plans on Thursday to clamp down on radical Web sites that attract the young and vulnerable to extremism.

Jacqui Smith will call for illegal material on the Internet to be removed during her first major speech on terrorism and radicalisation.

She will tell service providers: "Stopping people becoming or supporting terrorists is the major long-term challenge we face.

"We are already working closely with the communications industry to take action against paedophiles. I believe we should also take action against those who groom vulnerable people for the purposes of violent extremism."

She will add: "An effective response to terrorism depends on us - on the active commitment of individuals and communities to certain rights and responsibilities, to shared values which apply irrespective of religion or culture."

More than 500 million pounds will be spent on funding security and counter terrorism next year, rising to nearly 600 million pounds during the following two years.

The government will work with the Muslim community and Higher and Further Education colleges in an attempt to increase community cohesion.

But experts have expressed serious doubts about what can be effective to prevent radicalisation over the Internet, saying little research has been carried out.

Johnny Ryan, Senior Researcher at Dublin's Institute of International and European Affairs, has told Reuters that users could easily circumvent any restrictions imposed by the authorities.

Web sites could relocate from one country to another unless there was international agreement, while the controversial content was often distributed through services that are hard to block, such as legitimate chat rooms, he said.