"Laser louts" who shine high-powered beams of light at aircraft to distract pilots are being warned police are on their trail.

Officers in Leicestershire say the force's own helicopter is being targeted once every two to three weeks and they have warned offenders they face prosecution.

The crime of shining lasers at aeroplane and helicopter cockpits is am offence that has grown gradually in recent years as the pen-sized laser pointers have become more easily available online and in shops.

At first the police withheld details for fear of provoking copy-cats incidents but force policy has changed and officers have made frequent appeals, warning lasers could bring down an aircraft by dazzling pilots, endangering lives in the air and on the ground.

The beams can either pass straight through the Perspex cockpit and into the pilot's eye, or cause the inside of the helicopter to glow.

Mick Dunn, the police helicopter unit's executive officer, said while the problem was still serious, appeals had lowered the number of incidents slightly.

Police chiefs say they are determined to catch anyone who aims lasers at aircraft.

Mr Dunn said: "This is a country-wide problem and it's crass stupidity. I don't know why anyone would want to do it.

"It is distracting and some of our pilots have had to have their eyes looked at after incidents. Fortunately none of them suffered discomfort lasting more than a day or so.

"When the force helicopter is operating at 1,000ft or below the pilot is working hard to control the aircraft and a laser could bring it down.

"Our helicopters have equipment that can tell the crew where the light is coming from and it's not difficult to send a bobby on the ground to find who's responsible.

"That person will be prosecuted because the potential consequences are so severe."

A change to the rules at the beginning of this year has made prosecutions easier. Mr Dunn said: "We've had some convictions in Leicestershire and more are in the pipeline.

"In the past the prosecution had to prove the act actually put people's lives at risk, which was harder for juries to decide.

"Now shining a laser at an aircraft is an offence in itself. We don't have to prove the risk to life."

Commercial aircraft are also regularly targeted and the Civil Aviation Authority says it gets daily reports from pilots.

An East Midlands Airport spokeswoman said: "Targeting aircraft and distracting pilots with lasers is extremely irresponsible. We fully support any action taken by police to deal with the use of lasers."