Ninety per cent of the body’s vitamin D is produced by the action of sunlight on the skin.

And the researchers at the Medical University of Graz, Austria, said sufficiently high levels of the vitamin have a “positive influence” on testosterone, a major newspaper reported.

They found that men with high blood vitamin D levels have significantly higher testosterone levels than men with less vitamin D.

“Men who ensure that their body is at least sufficiently supplied with vitamin D are doing good for their testosterone levels and their libido,” said Ad Brand, of the Sunlight Research Forum, in Holland.