Online pornography is damaging the sexual health of young men, a senior psychosexual therapist has warned.
Men in their late teens and early 20s are increasingly likely to suffer erectile dysfunction, with experts blaming the problem on the spread of online porn.
Angela Gregory, a men's sexual health specialist at Nottingham University Hospital, said that young men who are addicted to watching graphic footage become desensitised and unable to be aroused by normal sexual behaviour.
And it is easier than ever to become addicted as pornography is so easily accessible on smartphones, tablets and laptops, she warned.
"What I've seen over the last 16 years, particularly the last five years, is an increase in the amount of younger men being referred," she told the BBC.
Erectile problems are most common in older men, particularly those with health problems affecting circulation.
But a 2014 study found one in four new erectile dysfunction patients was under 40.
Miss Gregory said: "Our experience is that, historically, men that were referred to our clinic with problems with erectile dysfunction were older men whose issues were related to diabetes, MS [multiple sclerosis] or cardiovascular disease.
"These younger men do not have an organic disease, they've already been tested by their GP and everything is fine.
"So one of the first assessment questions I'd always ask now is about pornography and masturbatory habit because that can be the cause of their issues about maintaining an erection with a partner."
In a BBC Newsbeat documentary examining the relationship between young people and porn, one man described how his addiction affected his life.
The man, known as 'Nick', started watching porn when he got his first laptop at the age of 15 and was soon watching it for up to two hours a day.
He said: "It quickly escalated and it was every day. What I was watching, it definitely got more extreme over a short period of time. There was nothing that would give me a kick.
"Normal stuff didn't do anything any more, so I had to get more and more extreme material. [It was] disturbing stuff that disturbed me."
He said he had trouble finding girls attractive as his sexuality "was completely wired towards porn"
Realising he had a problem, he spoke to his doctor who said she had seen a lot of young men come to her with similar issues.
It was only after he banned himself from pornography for 100 days that Nick was able to recover and have a normal sexual relationship.
In June a UK report found children as young as 11 were being "stripped of their childhoods" and warped by online porn.
A survey of more than 1,000 children by the NSPCC and the Children's Commissioner for England found 94 per cent of 14-year-olds had seen X-rated films or photographs.
Half of parents do not realise children who take nude selfies are breaking the law, according to an NSPCC survey. It also found two in five parents fear their children will be involved in sexting but most have not spoken to them about the risks.