THE informant told MI5 that they did not have much time to stop another lethal terrorist attack on London.

The details he passed on were so precise and so terrifying that intelligence agents had to drop some of their other investigations to concentrate on what was supposedly happening behind the net curtains of a neat terraced house in an East London suburb.

The belief was that the authorities had only days to act. Surveillance had to be hastily organised, the police and other agencies had to be told, along with ministers, that this time the terrorists were expected to use chemicals and not explosives to murder their victims.

The nervous informant claimed to have seen the chemical vest that the terrorist would use, and while he didn’t understand how the device would work, he did pass on a description and the address where he saw it. The man also offered a list of names.

Some elements of the story he had to tell agents bordered on the incredible, but security sources said that they dared not ignore this alert.

The trouble was that Operation Volga was happening at a time when Britain’s security apparatus was already dangerously overstretched. Bruised by recent public criticism of their conduct over the July 7 bombings, the police, intelligence agencies and senior ministers have deliberately let slip the scale of the threat facing Britain so that the country has some idea of the growing menace of Islamic extremists.

In a meeting last month with some families of victims of the July 7 attacks, John Reid, the Home Secretary, stunned his audience by telling them that 20 “major conspiracies” had been uncovered.

This was far more than anyone in Whitehall had previously divulged. Understandably, Mr Reid did not go into details but his claim came shortly after MI5 suggested that there were as many as 1,200 terrorist suspects living in Britain.

A report by the Joint Intelligence Committee leaked to a Sunday newspaper last month said that the war in Iraq had made Britain a target for al-Qaeda sympathisers “for many years to come”.

This report showed the remarkable rise in suspects that MI5 was attempting to shadow. At the time of the 9/11 attacks, MI5 knew of about 250 “primary investigative targets” inside Britain. By July 7 last year that had risen to 800. Today it is more than 1,000. Sir Ian Blair, the Metropolitan Police Commissioner, had revealed how three attacks had been foiled since July 7 that were likely to have caused many more deaths than the 52 killed in last summer’s suicide bombings.

Some of these plots were reportedly being put together by young, British-born Muslims. Finding these cells was the priority. But to add to the investigators’ workload, Tony Blair also ordered an operation to deal with foreign-born militants living and working here who were helping to organise and bankroll attacks overseas.

These included groups who were helping fanatics willing to take part in suicide attacks in Iraq against coalition forces. Raids by hundreds of police a fortnight ago that led to the arrest of eight men in Manchester and Merseyside was aimed at thwarting an al-Qaeda cell intent on targeting British and US troops.

At least seven men from Britain are believed to have been killed in Iraq since the insurgency began in 2003. Investigators have traced the travel routes these men took, the Islamic colleges that some cited on their visa applications to get to countries such as Syria, as well as the silent organisers in British cities who provide shelter and cash for these operations.

Mr Blair wants the Home Office to deport as many of these suspects as they can.

Ministers expect a tussle with the courts over this ambition as well as criticism from human rights groups over the Prime Minister’s message in the aftermath of July 7 that the “rules have changed” in the way Britain will deal with suspects.

Political opponents have claimed that MI5 leaking its ongoing inquiries is a ploy to block calls for a public inquiry into July 7.

Dame Eliza Manningham-Buller, the Director-General of MI5, said that her agents had suspended their work on some serious criminal cases to devote all their energies to counter terrorism.

While there have been efforts to share more intelligence with allies, this new generation of terrorists are more discreet than their predecessors. They no longer gather at mosques, where clerics rant against Western governments, or congregate with known militants. Instead they prefer to set up their own youth clubs, using back rooms in their parents’ houses to devise their schemes.

Today’s terrorists are suburban men who neighbours invariably describe as “hard-working, respectable and British to the core”.

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