Images of women such as Kelly Brook can be distracting to men

The more testosterone he has, the stronger the effect, according to work by Belgian researchers.

Men about to play a financial game were shown images of sexy women or lingerie.

The Proceedings of the Royal Society B study found they were more likely to accept unfair offers than men not been exposed to the alluring images.

The suggestion is that the sexual cues distract the men's thoughts, preventing them from focusing on their task - particularly among those with high natural testosterone levels.

The University of Leuven researchers gave 176 heterosexual male student volunteers aged 18 to 28 financial games to test their fair play.

They are looking for opportunities to pass on their genes

Dr George Fieldman, Buckinghamshire Chilterns University College

But first, half of the men were shown sexual cues of some kind.

One group of 44 men were given pictures to rate; some were shown landscapes while the rest were shown attractive women.

Another group, of 37 men, were either asked to assess the quality, texture and colour of a bra or a t-shirt.

And a third group of 95 were shown either pictures of elderly women or young models.

Each group was then paired up to play a game where the men had $10, a proposer had to suggest a split, and the other man accepted or rejected the offer.

If the second man accepted the offer, the money was distributed in agreement with the offer. If he rejected it, neither partner got anything.

The game is designed as a lab model of hunting or food sharing situations.

'Vulnerable'

The men's performance in the tests showed those who had been exposed to the "sexual cues" were more likely to accept an unfair offer than those who were not.

The men's testosterone levels were also tested - by comparing the length of the men's index finger compared to their ring finger.

If the ring finger is longer, it indicates a high testosterone level.

The researchers found that men in the study who had the highest levels performed worst in the test, and suggest that is because they are particularly sensitive to sexual images.

Dr Siegfried DeWitte, one of the researchers who worked on the study, said: "We like to think we are all rational beings, but our research suggests ... that people with high testosterone levels are very vulnerable to sexual cues.

"If there are no cues around, they behave normally.

"But if they see sexual images they become impulsive."

He added: "It's a tendency, but these people are not powerless to fight it.

"Hormone levels are one thing, but we can learn to deal with it."

The researchers are conducting similar tests with women. But so far, they have failed to find a visual stimulus which will affect their behaviour."

Dr George Fieldman, principal lecturer in psychology at Buckinghamshire Chilterns University College, told the BBC News website: "The fact men are distracted by sexual cues fits in to evolutionary experience. It's what they are expected to do.

"They are looking for opportunities to pass on their genes."

He said the study confirmed what had been suspected by many.

"If a man is being asked to choose between something being presented by an attractive woman and an ugly man, they might not be as dispassionate as they could be."