A policeman shows the thieves' smash and grab technique

Smash and grab thieves are taking advantage of partly open windows and sunroofs to get their hands on visible valuables, according to the ninth annual car crime survey by windscreen repair company Autoglass.

Dyfed-Powys Police has also reported a "marked increase" in car thefts at the Brecon Beacons, one of Wales' most popular leisure spots during bank holidays.

More family breaks have been ruined in Wales than anywhere else in the UK (45%), according to the research. In the UK last year, reported thefts from vehicles have risen from 629,651 to 655,127.

Since government statistics reveal that 48% of motorists do not report a break-in, the UK figure is over one million thefts with an estimated �800m worth of goods taken from cars.

Almost half of the car crime attacks in Wales have been on families with a typical loss of �310 per incident.

As a result, more than one in three families in Wales (36%) say they will leave their cars at home this bank holiday weekend, according to the survey.

'Great summer for crooks'

Cardiff is eighth in the UK hotspot list of thefts from cars, a list headed by Leeds.

Autoglass spokesperson Sarah Riggens said: "This has been a great summer for car crooks. When the weather is hot motorists are more likely to leave windows down and sunroofs open - an open invitation to thieves.

"Most of us are in a happy mood when the sun shines, but let's try to avoid giving Welsh car crooks something to smile about by being more alert with our vehicles and belongings this bank holiday weekend."

But many motorists in Wales are also angry that their vehicles are left vulnerable by poor security measures in public car parks where four in ten (42%) car crime attacks take place.

Almost nine in 10 people questioned in Wales (86%) said they would like to see parking security increased at leisure facilities, hotels and tourist resorts.

Thieves are targeting cars parked at beauty spots like the Brecon Beacons

The study also reveals that more than half a million UK family breaks have been spoiled by car crime this summer.

Ms Riggens advised motorists to park in a well-lit, open location for example under a lamp, in public view or close to CCTV cameras.

Valuables should not be left in the car or be locked away out of sight in the boot or glove compartment.

Stereos are the commonly stolen item. If detachable, the front panel of the radio, cassette or CD player should be removed.