Motorists will be targeted by a new generation of road cameras which work out how many people are in a car by measuring the amount of bodily fluid it contains.

The latest snooping device on the nation's roads aims to penalise lone drivers who abuse car-sharing lanes, and is part of a Government effort to combat congestion at busy times.

The cameras work by sending an infrared beam through the windscreen of vehicles which detects the unique make-up of blood and water content in human skin.

The system's inventors believe it will catch out motorists who try to fool existing CCTV road cameras by placing mannequins in passenger seats or fixing photographs to windscreens.

It will at first be used to police car-sharing lanes in Leeds, but councils across the country have already expressed an interest in using them.

Professor John Tyrer, who headed the Loughborough University team which created the device, said it would reduce congestion.

"It allows you to automatically count people," he said.

"That pools through to the congestion charging, so they can charge differently or reduce the rates dramatically if you've got more people in the cars."

But motoring organisations claim the cameras are a further intrusion on private lives and say car-sharing lanes - which are already in operation in Birmingham and Leeds and are being built on the M1 in Hertfordshire - do not work.

AA president Edmund King said: "Most of us work flexible hours. We don't go to work or come home from work at the same time.

"Car-sharing lanes are incredibly difficult to enforce and, if not many people use them, they're actually a waste of road capacity."

Roads Minister Rosie Winterton said last night she encouraged "innovative solutions" to the problems created by congestion.