Management at Peterborough's Queensgate Centre have installed a "Mosquito" alarm in the complex, which is set off when youths start to congregate.

The ultrasonic device – which was installed in mid-June and will be trialled until the end of the year – emits an irritating noise, which is usually only heard by those under the age of 20.

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So far, the scheme is believed to have been successful in dispersing groups and slashing instances of anti-social behaviour.

Queensgate manager Roger Hutchings said: "The scheme is currently still under trial and appears to be a success so far.

"We have seen a reduction in the amount of people hanging around the entrance areas of the centre, although this may also be to do with the introduction of the smoking ban."

He said the device – dubbed by some as the "teen tormentor" – was not activated constantly, only at certain times when groups started to congregate.

The alarm is switched on, on average, fewer than 10 times a week, for a few minutes at a time.

City police say they, too, have noticed a down-turn in incidents since the introduction of the alarm.

City centre Sergeant Mick Thorpe said that in the first few months of the alarm being installed, there had been a noticeable reduction in the number of incidents.

He said: "Basically, they find the noise emitted by the alarm irritating.

"It definitely seems to be working."

The Mosquito alarm was devised by the Welsh company, Compound Security Systems.

It draws on the fact that due to a medical phenomenon known as presbycusis, or age-related hearing loss, people who are older than 20 tend not to be able to hear certain high-pitched sounds.

"The company says the device has been praised by forces across the country for its deterrent effect, and is bought not only by shopkeepers, but also by rail companies which want to deter yobs from scrawling graffiti on railway bridges and property."

Spokesman Simon Morris said: "The noise is modulated and flicks on and off four times a second so you can't block it out. This is why it is so irritating.

"It is like your alarm clock – you could perhaps stand it for a minute or so, but then you would have to go and switch it off."