The Home Secretary told MPs that he was not considering a broad hierarchy of “green, amber and red” alerts as adopted in America. “Without being facetious, some of us are old enough to remember Orson Welles,” he said. “We have to be very careful that we don’t trigger anything like that.”

Mr Blunkett was referring to the radio version of H.G. Wells’s novel, broadcast in 1938. Its documentary style was so realistic that people across America believed the planet had been invaded by Martians. Some listeners fled for their lives; others flooded the radio station with calls.

Mr Blunkett told a joint session of the select committees on home affairs and defence: “We took the view that constant public reports on the changing levels (of risk) would not be helpful.”

Mr Blunkett was asked what measures the Government had taken for the evacuation of towns and cities in the event of a mass terrorist attack. Pointing out that terrorists aimed to cause fear and damage economies, as well as kill people, he said: “Evacuation plans are part of the preparation . . . and we would have these plans in place. But they have to be balanced in relation to what would happen if panic ensued.”

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