Internet porn offering a seemingly endless choice of sexual images may be fuelling sex addiction, say researchers.

A study found that sex addicts are more driven to seek out new sexual images than other individuals.

While there is no suggestion that it triggers sex addiction, the abundance of sexual images available online is likely to worsen the problem for those affected, according to the authors.

Sex addiction, defined as an inability to control sexual thoughts, feelings or behaviour, affects as many as one in 25 young adults.

A number of celebrities have reportedly battled sex addiction, including comedian Russell Brand, actor David Duchovny, and golfing star Tiger Woods.

Dr Valerie Voon, from Cambridge University, who led the new study of 62 men including 22 diagnosed sex addicts, said: "We can all relate in some way to searching for novel stimuli online - it could be flitting from one news website to another, or jumping from Facebook to Amazon to YouTube and on.

"For people who show compulsive sexual behaviour, though, this becomes a pattern of behaviour beyond their control, focused on pornographic images."

She added: "Our findings are particularly relevant in the context of online pornography.

"It's not clear what triggers sex addiction in the first place and it is likely that some people are more pre-disposed to the addiction than others, but the seemingly endless supply of novel sexual images available online helps feed their addiction, making it more and more difficult to escape."

Participants in the study, published in the Journal of Psychiatric Research, were first shown a series of paired images including naked and clothed women, and furniture.

They were then shown another set of image pairs, some of which were familiar and some new, and asked to choose a picture "to win £1". There was a 50/50 chance of picking the right image and winning the prize.

Sex addicts - but not volunteers who were not addicted to sex - were more likely to choose novel over familiar sexual images, the researchers found.

In a further test, 20 sex addicts and 20 non-addicted volunteers underwent brain scans while being shown repeated images of a nude woman, a £1 coin or a neutral grey box.

When the sex addicts viewed the same sexual image over and over again, they experienced a greater reduction of activity in the dorsal anterior cingulate cortex, a brain region known to be involved in anticipating rewards.

This was consistent with the "habituation" that occurs when an addict finds the same stimulus less and less rewarding.

The finding suggests that to prevent this happening a sex addict would need to seek out a constant supply of new images, said the authors.

Another part of the study indicated that apparently innocuous environmental cues could spark a sex addict's quest for stimulating sexual images.

"Cues can be as simple as just opening up their internet browser," said Dr Voon. "They can trigger a chain of actions and before they know it, the addict is browsing through pornographic images.

"Breaking the link between these cues and the behaviour can be extremely challenging."