The prime minister has given his backing to a warning by the head of MI5 on the scale of the terror threat facing the UK.

Tony Blair said the government was "looking again" at new counter-terror laws.

Dame Elizabeth Manningham-Buller said the security service is investigating 30 major British-based terrorist plots, more than 1,600 suspected terrorists and 200 terror networks.

Home secretary John Reid is thought to have given Dame Eliza clearance to tell an invited audience of academics on Thursday that the threat would last "a generation".

The normally low-profile security chief was speaking ahead of next week's Queen speech, which is expected to contain new counter-terrorism legislation.

Dame Eliza said some of the "mass casualty" terror plots involved suicide attack and some we being directed by al Qaeda.

And she voiced concern that there is widespread sympathy for terrorism in Britain.

"If the opinion polls conducted in the UK since July are only broadly accurate, over 100,000 of our citizens consider that the July bomb attacks in London were justified," she said.

She said young people were being actively groomed, and was alarmed at the "scale and speed" of their radicalisation following the London bombings.

"[The] threat is serious, is growing, and will, I believe, be with us for a generation," the MI5 chief added.

"It is a sustained campaign, not a series of isolated incidents. It aims to wear down our will to resist."

And the prime minister gave his backing to Dame Eliza's comments, saying: "This is a threat that has grown up over a generation.

"I think she's absolutely right in saying it will last a generation and it can only be combated in the end not just with proper measures of security, and we're looking again at how we can strengthen our terrorism laws.

"But in addition to that we have to take on and combat the poisonous propaganda of those people who warp and pervert the minds of particularly young people."