The Tory leader's penchant for removing his helmet when he comes in sight of a camera has infuriated MPs, who say he is setting a terrible example to children. They have seized on his helmetless appearance at the Commons on Budget day and urged him to change his ways and put his safety before his hairstyle.
The Conservative leader, who often cycles into work on his mountain bike, is not the only bike-riding Tory MP to be singled out. Boris Johnson, the party's flamboyant higher education spokesman, and Andrew Mitchell, shadow international development secretary, also reportedly fail to wear helmets.
Denis MacShane, Labour MP for Rotherham, who cycles in every day, has written to Mr Cameron urging him to set a good example.
"I have myself been knocked off my bicycle while navigating approaches to Parliament," he said.
According to the Royal Society for the Prevention of Accidents, around 75 per cent of cyclists killed have major head injuries.