Muslim children as young as 15 are being recruited by al-Qa'eda to wage "a deliberate campaign of terror" in Britain, the head of MI5 has said.

Jonathan Evans used his first speech since taking over the security service to warn that Islamists were "radicalising, indoctrinating and grooming young, vulnerable people to carry out acts of terrorism."

Mr Evans said the threat has yet to reach its peak

He said MI5 had identified 2,000 individuals who pose a direct threat to national security and public safety but warned the number of potential terrorists living in this country could run to 4,000.

The stark assessment came as the Government prepared to announce plans for a new counter-terror crackdown.

The Queen's Speech on Wednesday will foreshadow a Bill to make it easier for police to interrogate terrorist suspects.

Harsher sentences for terrorist-related offences are also expected and Ministers want to increase the limit for holding suspects without charge beyond the current 28 days.

The legislation, to be published before Christmas, will for the first time allow police to continue questioning detainees after charging them.

Mr Evans, who took over as the director general of the security service in April, said police and MI5 have been conducting a covert inquiry, known as project Rich Picture, to find people being groomed for terrorism and pinpoint the Islamic extremists carrying out the recruitment.

They also have set up eight new counter-terror units (CTUs) around the country to expand their coverage.

As a result they have now identified the 2,000 individuals - an increase of 400 in a year - but Mr Evans said the number of individuals that could pose a threat to national security but have not come to the attention of police or MI5 could be double that figure.

Mr Evans said: "This increase is due both to improved coverage of extremist communities and to the continued flow of new recruits to the extremist cause."

He added: "Extremists are methodically and intentionally targeting young people and children in the UK.

"This year, MI5 has seen individuals as young as 15 and 16 implicated in terrorist-related activity.

"The UK needs to protect its children from exploitation by violent extremists."

Mr Evans said the threat posed by al-Qa'eda had yet to reach its peak despite one successful bomb attack and several thwarted atrocities.

Mr Evans said the successful and attempted attacks in Britain had not been random plots by disparate groups but co-ordinated and deliberate actions ordered by al-Qa'eda's leadership.

"The majority of these attacks, successful or otherwise, have taken place because al-Qa'eda has a clear determination to mount terrorist attacks against the United Kingdom," he said. "This remains the case today, and there is no sign of it reducing.

"Al-Qa'eda is conducting a deliberate campaign against us. It is the expression of a hostility towards the UK which existed long before September 11, 2001."

Mr Evans said terrorist plots were becoming increasingly difficult to monitor and to crack.

"We have seen unsophisticated attempts to kill and injure, but we have also seen complex, logistically effective plots, which require a high degree of expertise and accurate targeting," he added.

Ministers say the principal reason why longer detention is needed to question suspects is the growing complexity of the conspiracies now being uncovered.

But the Conservatives and other opposition parties say there is no evidence to show a longer detention period is needed.

So far, none of the suspects held in a variety of plots have had to be released because police had run out of time.

However, the Government - backed by Lord Carlile, the independent reviewer of counter-terror legislation - say a longer period, perhaps 56 days, is needed as a "precaution".

David Davis, the shadow home secretary, said Mr Evans's speech did not substantiate the Government's case.

"As Mr Evans warns in clear terms, we must avoid an "indiscriminate response" that would drive young Muslims into the arms of fanatics and destroy the trust of local communities. We agree," he said.

Nick Clegg, Liberal Democrat home affairs spokesman, said it was important that the security services did not "get drawn into politics".

He added: "Experience suggests that breathless talk about the terrorism threat and the powers needed to counter it can have a damaging effect on opinion in precisely those communities that we need to keep on our side."

Shami Chakrabarti, the director of Liberty, said: "Extending to 56 days will make us an embarrassment to the world. What would we say to the Burmess generals? To Robert Mugabe?

"At 28 days we are the worst in Europe and the worst in the western democratic world. That is not right for the land of Magna Carta."