Most were living quietly with their parents or their young families. Bemused neighbours of the men said that that they had lived at the same addresses for years and had jobs such as taxi drivers and builders; one was an airport caterer. Another of the teenagers arrested yesterday was a student with ambitions to go to university.

All the young men were described as models of suburban respectability. But counterterrorist officers and MI5 suspected otherwise.

“This was truly the enemy within,” said one senior figure involved in what was named Operation Crevice. “This is proof that it’s not a question of if, it’s the when and the where.”

Even as leading politicians argued on television whether the public should be scared by repeated warnings from police and ministers about the inevitability of a terrorist strike, the plot was fast taking shape.

David Blunkett, the Home Secretary, was informed about it and gave permission for the telephones of some of the suspects to be tapped.

While all the speculation in recent days has been of possible British links to the train bombings in Madrid, the intelligence agencies have been concentrating on a number of terraced houses in locations such as Crawley, Slough and Ilford.

Undercover teams had been closely shadowing some of those arrested yesterday and are reported to have linked them to others in the group through telephone calls and e-mails.

At this stage, the counterterrorist teams said they did not know how any attack was to be carried out. Most of the men picked up yesterday were considered too young to have fought in Afghanistan or to have been schooled in bomb-making at al-Qaeda training camps. Police believe that they were recruited in Britain. Although most of them are of Pakistani origin, all were born in Britain or have spent most of their lives here.

The focus of the inquiry suddenly changed with a string of intercepted telephone calls inquiring about renting space in storage warehouses. These anonymous, prefabricated buildings are the perfect hiding place. They are large enough to store vehicles and, as witnessed yesterday, a builder’s sack full of industrial- strength fertiliser, without anybody paying much attention.

There are a number of Asian-owned building firms that use the Access storage centre in Hanwell where the fertiliser was found, so the sight of young men lugging a 6ft bag of what looked like builders’ materials was not out of the ordinary.

The dilemma for the security authorities was when to move in. Operation Crevice differed from previous terrorist surveillance operations in that the men being watched were spread so widely around London and the Home Counties.

Detectives were understandably guarded about why they chose yesterday to make their move. One suggestion is that they intercepted a telephone call which indicated that the half tonne of fertiliser was about to be moved.

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