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

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

Within 24 hours anti-terror police were heading to Glasgow searching for members of the terrorist cell. They warned the largest shopping centre in the area to increase security in its car parks.

The Sim card gives each handset its own unique number and it is believed that the terrorists, believing the phones would be destroyed, left several crucial numbers in the telephone’s memory.

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One of these may have been a landline, giving officers a name and address and a place to start their search. Every number stored in the telephone would be subjected to intense investigation with police tracking down each owner where they could and pinpointing where and when each call was made.

A police source told The Times: “All Sim cards have a record of the calls that that telephone has made or received and you can then work out 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 Glasgow airport was attacked by two men who drove a Jeep into the front of the building.

Then 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 Mercedes cars.

As the car drove north the telephone would register 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 tracking the car before making their move. Armed officers would drive side by side in two lanes with another car hanging back on the fast lane to allow normal 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 slowly gain on the suspects.

Once at least three cars had passed the target car they would slow down and wait for three others to come up behind and each side of the car.

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

Mick Shelley, a digital forensic analyst, for Focus Forensic Telecoms, 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 north east direction for example and then plot that route with a motorway or train service.

“If it is off it has to leave a mark which will tell police where it was turned off.”

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