Last updated at 17:22pm on 9th March 2008
Warning: Terror moles reportedly work in the Met
MI5 are said to have homed in on the the "sleeper" agents passing secrets from Scotland Yard to the terror group only in recent weeks.
The suspected spies are believed to have used methods similar to those employed by the IRA in the 1970s as they infiltrated the police and the Army in Northern Ireland.
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MI5 chiefs reportedly believe the suspected moles have been planted as sleepers - agents under deep cover - to keep al-Qaeda informed of anti-terror raids planned by London's Metropolitan Police.
They are said to fear the four could have already accessed sensitive information about secret operations to root out terror cells planning further attacks in the UK.
Scotland Yard refused to discuss allegations that a sleeper cell of al Qaida spies is being kept under secret service surveillance after infiltrating the Metropolitan Police.
According to the News of the World, in the past few weeks MI5 agents have identified four officers suspected of passing secrets from the force.
All four are allegedly Asians living in London and are feared to have links with Islamic extremists in Britain and worldwide terrorist organisations.
MI5 bosses reportedly fear the moles may have been planted to keep al Qaida informed of anti-terror raids, and may have already accessed sensitive information about ongoing operations.
Secret service agents are said to be monitoring the suspects, who work at different London police stations. A Yard spokesman said: "All police officers and police staff, upon joining the Metropolitan Police Service and during their careers undergo a range of security checks.
"These are robust and vary accordingly to the type and sensitivity of the individual postings.
"We take matters of security very seriously and if any issues arise about individuals, they may be subject to further assessment.
"This could lead to restrictions being put in place relating to where an individual may work within the organisation or could lead to their dismissal."
"If there are people within the police force feeding information to terror groups this needs to be stopped.
"Since the names came to light there has been a non-stop effort to find out everything about their backgrounds."
The officers' names apparently emerged during a low-profile investigation into police force infiltration which has been going on since the July 2005 London bombings.
Last year MI5 believed there were up to eight police staff—uniform and civilians with links to extremist groups.
Now agents, helped by anti-terror police, are understood to be watching the four suspects - who work at different police stations around London - around the clock while searching for the vital evidence needed to make arrests.
The officers' every move at work is being monitored along with their phone calls, it was claimed.
Homeland security agents are reportedly sifting through their bank account transactions.
MI5 experts are also understood to be building a family tree for each one and trying to put together a picture of their links to their home countries.
Their names are being cross-referred with lists of men who have been to terror training camps in Pakistan or Afghanistan.
What is clear is that the infiltration methods used by the officers under suspicion bear hallmarks of the IRA in the past.
The police source said: "The IRA tried to infiltrate and they succeeded to a certain extent.
"By just slipping under the radar it takes suspicion away from you.
"If you are a young Pakistani of English origin and you feel you want to do something for the cause of Islam, what better way than to join the enemy and attack from within?"
MI5 believes other sleeper cells are trying to infiltrate public services across Britain in order to gain vital intelligence.
Even exiled cleric Omar Bakri has revealed how Islamic extremists were working at the heart of the NHS and other vital services.
Failed asylum seeker Omar Altimimi was jailed for nine years last July for keeping manuals on detonating car bombs.
Before his conviction he had applied to work as a cleaner for Greater Manchester police.
Numbers of officers from ethnic minorities have risen since the Met was accused of being institutionally racist in the Stephen Lawrence public inquiry report.
MP Patrick Mercer, Tory terrorism advisor, said: "This discovery by MI5 comes as no surprise to me.
"Recruiting ethnic people into key public sector organisations - in place to protect us - is a risk.
"Our vetting procedures have to be toughened before it's too late."