Police are tightening the screw on car criminals with a special device to keep number plates secure.

Officers were today fitting thief-proof screws to Bradford registration plates at the start of a new campaign - and asking for help in solving a mystery.

Since last June, around 230 number plates have been removed from cars in the Bradford North division and thefts are currently running at the rate of two a day.

A third of the stolen plates are fitted to cars for use in crimes. And a small number of drivers remove them themselves - to try to avoid prosecution after being caught by speed cameras.

But more than half the stolen plates are dumped or simply disappear. Detective Inspector Noel Devine, of Bradford North CID, said: "We have no idea where or why and we need the public to help solve the mystery. We want to hear from anyone who knows what is happening to them."

The new screws can only be attached and removed with a special key, making plates tamper-proof for thieves who would normally use a screwdriver.

The Bradford North and Bradford West Anti-Crime Partnerships have provided funding for police to hand out 250 of the special screws and could provide more.

The scheme was being launched today at Morrison's supermarket in Thornton Road, with similar sessions planned over the next three weeks.

Det Insp Devine said: "This is an ongoing problem that the police in Bradford are committed to tackling." He said that in the last week alone there had been 12 registration plates stolen in the division.

"We had thefts in Heaton, Girlington, Eccleshill and Idle, but there are no particular hotspots," said Det Insp Devine. "We have had them reported in car parks and outside homes. There is no pattern."

Information and advice on how to avoid being a victim of vehicle crime was being offered from the Bradford North Crime Reduction Bus at today's launch.

Further events will be held at the same venue next Thursday and at Morrisons, Thornbury, on February 2 and 9.

Anyone with information about the theft of number plates should ring Lawcroft House police station on (01274) 376259, or Crimestoppers anonymously on 0800 555 111.