Al-Qaeda is actively "grooming" children and young people to carry out attacks in Britain, the head of the country's domestic intelligence service said.

Jonathan Evans, in his first public speech since becoming MI5 chief in April, said Britain was facing "the most immediate and acute peacetime threat" that his century-old agency has ever known.

"As I speak, terrorists are methodically and intentionally targeting young people and children in this country," the security service head said.

"They are radicalising, indoctrinating and grooming young, vulnerable people to carry out acts of terrorism. This year, we have seen individuals as young as 15 and 16 implicated in terrorist-related activity."

Speaking to the Society of Editors Conference in Manchester, north-west England, he said the number of individuals in Britain with suspected terrorist links rose from 1600 last year to at least 2000 this year.

And he insisted many were co-ordinated as part of a "deliberate campaign" by al-Qaeda against Britain.

"Terrorist attacks we have seen against the UK are not simply random plots by disparate and fragmented groups," he said.

"The majority of these attacks, successful or otherwise, have taken place because al-Qaeda has a clear determination to mount terrorist attacks against the United Kingdom.

"This remains the case today, and there is no sign of it reducing."

Britain, the United States' main backer in the invasions of Afghanistan and Iraq after the September 11, 2001 attacks in Washington and New York, has long been on high alert for attacks.

On July 7, 2005 four suicide bombers killed themselves and 52 other people in co-ordinated attacks on the London transport system.

In June this year there were failed car bomb attacks in London and Glasgow.

AFP