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Nylon tights were used to mask the emergency lights on an unmarked police car involved in a crash, the police watchdog has said.

The North Wales Police officer driving the BMW was seriously injured when his car hit a lorry in a lay-by.

An investigation by the force found the tights covering the front grille significantly reduced the light output.

The Independent Office for Police Conduct (IOPC) said there should be a national policy on covering lights.

The police probe found the officer had turned into the lay-by to avoid a vehicle which moved into his lane on a dual carriageway.

It discovered the blue lights in the front grille had been covered with tights to help prevent members of the public identifying it as a police car when the lights were not being used.

The inquiry concluded the tights could not be ruled out as a contributory factor to the crash.

It said they made it extremely difficult for the driver of the vehicle on the dual carriageway to see the flashing lights.

The subsequent IOPC investigation identified no national standardisation for covering lights and, as a result, a number of police forces were using various methods - including nylon tights.

Catrin Evans, the IOPC's director for Wales, said: "Any modifications made ought to be tested and approved by experts rather than using ad hoc solutions that may not be the safest method.

"For the safety of police drivers and the public alike, testing would also help make sure that any coverings do not significantly limit the visibility of the emergency blue lights."

The IOPC's recommendations have been accepted by the National Police Chiefs' Council which has now commissioned a working group to produce the new policy.

The watchdog found no case to answer for the officers involved in covering the grille lights.

No members of the public were hurt in the crash.