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Most of us have experienced it - a friend warns us about a new speed camera or trap, advising us to be cautious when driving nearby.

But one driver learnt that warning other motorists of a police speed trap is a criminal offence - the hard way.

The driver was “waving frantically” at other motorists after passing a mobile speed camera on the A30 near Launceston, CornwallLive reports .

But, unfortunately for him, one of the drivers he ‘warned’ happened to be a police officer in an unmarked car.

Posting on the Devon and Cornwall Police No Excuse Twitter page, an officer wrote: “Driving and waving frantically at our unmarked vehicle to ‘warn’ us of the camera van on the #A30 is not the smartest move.

"The excuse was that he doesn’t like speeders but he doesn’t want them being ‘caught’ either! Driver reported for the offence.”

The officer told CornwallLive that the driver was reported for obstructing the police, which carries a maximum penalty of one month's imprisonment and/or a fine of up to £1,000.

The officer added: “On a serious note, that kind of behaviour not only hampers the good work that staff and officers do to reduce speeding motorists (one of the Fatal Five) but their actions are also reckless as it can result in other motorists slamming on their brakes & causing an accident.”