A gadget which emits a high frequency noise audible only to young people could be the latest weapon in the fight against anti-social behaviour in Hertsmere.

The Mosquito alarm, which releases a high-pitched noise that only people under the age of 25 can hear due to its frequency range, is set to be piloted in Borehamwood and Radlett in a bid to combat youths who loiter in large groups, causing nuisance to shops and residents.

The device, which costs around £500, works by emitting an ultrasonic tone which becomes so annoying, it encourages the youngsters to move away from an area and disperse.

The closer one gets to it, the more effective it is.

Trials in other parts of the country so far have shown teenagers are acutely aware of the Mosquito and move away from an area in just a couple of minutes.

Charlotte Weston, leader of Elstree and Borehamwood Youth Council, said the devices would discriminate against everyone under 25.

She said: "I think it's awful. They are branding us as animals. I'd love to meet the person who came up with this idea. I am shocked. They can only install it if they put one in for over-25s as well, I'm dead against it we are not pigeons."

But Hertsmere crime prevention officer Chrissy Barclay has high hopes for the device, which has undergone months of health, safety and technical trials.

"There's only so much you can do to deter nuisance youths from hanging round causing trouble on the streets," she said. "This is a completely new and pioneering piece of equipment and one which will hopefully help our police and police community support officers tackle the problems associated with anti-social behaviour from noise nuisance to drug-taking.

"It's unlike anything we've ever used before. It's another tool in our armoury."

The Hertsmere crime and disorder reduction partnership (CDRP) will fund and oversee the running of the alarms, which will be tested over a three-month period. The borough council will now carry out extensive risk assessments in hotspots across the borough prior to installing the alarms to ensure they do not cause any problems to residents living nearby.

Chairman of the CDRP, Councillor Morris Bright, said: "This is a unique way of dealing with some of the problems of nuisance caused by young people.

"However, let's make no mistake that not all anti-social behaviour is caused by young people and I want to ensure that youths aren't stigmatised by the behaviour of a few.

"Nevertheless, I do welcome any moves that sensibly cut down on low-level crime in our towns and communities."