Hormones produced by raised stress levels have been found to aggravate acne.

More than 85 per cent of people suffer the disorder at some point in their lives, although scientists are still unsure exactly what triggers it.

Researchers from Stanford University studying 22 students with varying degrees of acne found the condition deteriorated during stressful times, such as the run-up to exams. The link remained even when other factors such as sleeping hours and diet were taken into account.

Writing in the journal Archives of Dermatology, the researchers said that while stress may not cause acne, it can exacerbate it. Various mechanisms have been proposed for why stress may aggravate acne. Some investigators believe stress stimulates the release of hormones known to worsen acne by increasing production of oily substances from sebaceous glands found in the skin.

There is also research which suggests that stress increases production of chemicals that can trigger inflammation, and stress is known to slow the wound healing process by up to 40 per cent.

Dr Niall Wilson, a dermatologist at the University of Liverpool, said: "Lots of patients describe stress as a factor which flares their skin. It is by no means the only factor - lots of things interact."

Research has found that stress may exacerbate symptoms of other skin diseases, such as psoriasis.

There are more than 2,000 skin diseases affecting eight million people in Britain. But acne is the most common, with teenagers hardest hit. About one in three of those affected continues to suffer from the condition later in life.

Earlier this year, scientists revealed that pilot studies into the effects of laser treatment on acne resulted in a reduction of the condition by as much as 50 per cent.