Several people arrested in connection with the terror attacks were traced after mobile telephones found intact in the failed London car bombs yielded crucial information.

As soon as police recovered the phones from the two Mercedes cars, officers went to work on the Sim cards, gathering a wealth of intelligence from the numbers stored.

Within 24 hours antiterrorist police were heading to Glasgow searching for members of the terrorist cell and warning the largest shopping centre in the area to increase the security in its car parks.

The Sim card gives each handset its own unique number, from which police can tell whether it is a pay-as-you-go phone or is on a contract. A contract phone would allow police to find out the owner’s name and address, while pay-as-you-go gives no such information.

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It is thought that the terrorists, believing that the phones would be destroyed, were lazy and left several crucial numbers in their telephones’ memories. One of these may have been a landline, giving officers a name and address to start their search. Every number stored in the telephone would be subjected to intense investigation, with police tracking down each owner where possible, and pin-pointing where and when each call was made.

A police source told The Times: “All Sim cards have a record of the calls that telephone has made or received and you can then work out at what location they were received or made from. If one of the stored numbers happened to be a landline, then that would give us the name of a registered owner and an address.”

Officers arrived at an address in Glasgow, a few miles from the airport, at lunchtime on Saturday, a few hours before the airport was attacked by two men who drove a Jeep into the front of the building.

Late on Saturday night they arrested Dr Mohammad Asha and his wife, believed to be called Dana, as they drove along the M6 in the northbound carriageway near Sandbach, Cheshire, 12 miles from their three-bedroom home in Newcastle-under-Lyme, Staffordshire. The couple could have been tracked by their own mobile telephone, whose number may have been retrieved from one of those found in the Mercedes cars.

Authorities could also have been alerted by the automatic number plate recognition (ANPR) system, which checks all registrations with a national database of wanted vehicles.

As the car drove north, the telephone registered with telephone masts on the route, each one handing the telephone on to the next when it lost the signal. Every time a telephone connects or breaks from a mast a record is made, allowing officers to track its route.

At least nine cars would have been involved in tailing the car before police made their move. Armed officers would have driven side-by-side in two lanes with another car hanging back in the outside lane to allow traffic to pass. When the target car was in sight three of the police cars would slow down, not allowing traffic to pass, while the other unmarked cars would gain slowly on the suspects. When at least three cars had passed the target car, they would slow down and wait for three others to come up behind and on each side of the car.

As soon as all the cars had boxed in the suspects, the lead car would stop and put on its flashing lights, giving the signal for armed officers to get out and surround the car.

Mick Shelley, a digital forensic analyst for Focus Forensic Telecommunications, said: “The brain of the telephone is the Sim card, which holds all the data. Once police have this they will be able to get to all the numbers stored and track down the owner by working backwards – maybe one of the calls to him is from a landline and they will get the name through the owner of the landline.

“They will also be able to track the telephone, if it is on, by following the telephone masts it passes. They may not be able to say exactly where but will be able to work out if it is travelling in a northeast direction, for example, and then plot that route with a motorway or train service. If it is off, it has to leave a mark that will tell police where it was turned off.”

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