Revealed: Islamist extremists have penetrated the heart of Britain

Counter-terrorism officials say "insiders" or their associates are almost certainly working "undetected" in sensitive posts and are actively supporting the activities of extremists.

In some cases, lifelong relationships between friends or relatives are being exploited to obtain crucial information from those in sensitive posts.

The development is detailed in intelligence reports circulated to the Home Office, police and Whitehall officials.

The London Underground, Gatwick airport and BT are cited as examples of organisations which have been targeted by individuals linked to terrorists.

Officials say the idea of "penetrating the enemy is pervasive" for Islamist extremists.

It is understood a number of suspected jihadists working in Government departments and the public services are being monitored by the security services.

Details of the threat emerged months after the Daily Mail revealed fears that Scotland Yard has been infiltrated by individuals linked to extremist groups including Al Qaeda.

Several police officers and civilian staff are being monitored amid claims they are long-term sleepers trying to gain sensitive information of use to terrorists.

Some are even believed to have attended terror training camps in Pakistan or Afghanistan.

Fanatics who infiltrate the Government or the "Critical National Infrastructure" - vital utilities such as water, electricity, transport and communications - have a number of objectives.

These include trying to gain information on what the law enforcement agencies know about the activities of fellow Islamist extremists and how to evade the attention of police and the security services.

They may also try to obtain information or intelligence to help them to carry out acts of terrorism.

This involves getting access to premises or individuals "with the immediate purpose" of mounting an attack or obtaining sensitive information to facilitate a later atrocity.

The extremists might also seek information which is of "indirect use" to the planning of a terrorist attack - such as getting access to banking information to raise money through fraud, gaining insider knowledge about airport security and surveillance measures on the London Underground.

Security sources say there is evidence that UK-based terrorists have discussed the possibility of attacking national infrastructure targets with the help of a "sympathetic insider".

MI5 has warned in the past that suspects with "strong links" to Osama bin Laden have tried to join the British security services and, in January last year, exiled radical Omar Bakri claimed that Islamist extremists were infiltrating the police and other public sector organisations.

College teachers must be "vigilant" in tackling the threat posed by violent extremists who attempt to recruit teenage students to terrorism, ministers said yesterday.

Al Qaeda supporters seek to "groom" impressionable young people and staff should be prepared to tell the police if they have concerns, draft Government guidance said.

The guidance, published for consultation, is aimed at colleges teaching students aged 14 and over, including more than 700,000 aged 16 to 18, and follows similar guidelines for universities.

The terrorist traffic warden

A terrorist jailed for his involvement in a bomb attack on the Paris Metro later came to England and got a job as a traffic warden.

Mustapha Boutarfa, 32, was arrested by Scotland Yard's anti-terrorist squad in 1996 and extradited from Britain to France two years later.

He stood trial for his auxiliary role in the 1995 attack on the St Michel station by a notorious Islamist militant group, in which eight were killed and 80 wounded, and was given a two-year prison sentence.

But after his release, Boutarfa, who held dual French and Algerian nationality, managed to get back into the UK with his wife and children and secured the job as a parking attendant in Richmond-upon-Thames, Surrey, with NCP Services.

Boutarfa's secret would probably never have come to light had he not accused a van driver of assaulting him in a row over a parking ticket in October 2005.

It led to the sensational disclosure about his past in open court.

That case was then dropped and he was charged with fraud but walked away with a 12-month suspended sentence. He has quit his job.

It is still not clear how he was allowed back into Britain.