Gerbils can scent increased adrenalin from sweat - an instinct which makes them the perfect detector of people up to no good. Or so security boffins thought.

The wheeze was disclosed by Sir Stephen Lander, the head of MI5, at a conference for academics at the public record office in Kew, south-west London. A hitherto unnoticed file in the archives refers to the "security use of gerbils", Sir Stephen said. What he did not disclose is that the wrong kind of people would be arrested.

The idea that the small furry pets could be employed as spycatchers was first mooted by Canadian scientists in the 1970s. The plan was to place a cage of gerbils by the immigration desks at airports. On the other side of the queue of passengers, an electric fan would discreetly blow the scent of excess sweat in the direction of the cage. The gerbils would get excited and in a trained Pavlovian response they would push a lever, pointing the (metaphorical) finger of suspicion at the suspect.

Most national security agencies were wary of such a scheme. It was, however, put into operation by Shin Bet -the Israeli internal security service - at Tel Aviv airport.

Until, that is, it found that instead of catching spies or terrorists, the gerbils caught innocent victims who were giving away their terror of flying.

British security sources also pointed out that the gerbils could not distinguish between sweat produced by a nervous spy and passengers suffering from carrying heavy baggage.

The gerbil trap proved less successful than other schemes featuring in an exhibition at the PRO, Shaken not Stirred. The exhibition, which opens today, describes how MI5 trained peregrine falcons to intercept enemy pigeons used to convey secret messages.