The system of cameras, sited on trunk roads and motorways, can follow a car’s movements over hundreds of miles and automatically alerts police if a car is stolen, if road tax has not been paid, if the MoT is out of date or if the driver is uninsured.

This would have allowed detectives to track two suspects as they travelled north on the M6 and then intercept their car.

The daring high-speed chase was carried out by the Metro­pol­itan Police’s Counter Terrorist Command – supported by officers from West Midlands Terror­ism Unit. Experts said some of the best clues to the terrorists’ identities would have come from mobile phones found in the two Mercedes.

Using mobile phone cell site analysis, detectives were able to pinpoint the precise area where calls were made or received.

A security source said: “If the bombs had gone off, the cars would have blown into a million pieces and the task facing the police would have been enormous. As it was, evidence, particular cell site analysis, could be processed very quickly with the possibility of making arrests very much enhanced.”

The two Mercedes also yielded up DNA samples, fingerprints, and documents. Strath­­clyde Police yesterday appealed to anyone with information about the green Jeep Cherokee used in the attack at Glasgow airport to contact them.

They particularly want to know where the vehicle – registration number L808 RDT – was kept over the past days and weeks.

Deputy Assistant Commissioner Peter Clarke, head of the Met’s Counter Terrorism Comm­and, said he was “absolutely confident” that his detectives would gain a thorough understanding of the terrorists’ methods, plans and networks. He said: “This investigation is extrem­ely fast-moving. New information is coming to light hour by hour.”
Police said anyone with information should ring the Anti-Terrorist Hotline on 0800 789 321.