A controversial Channel 4 documentary has caused outrage with its depiction of a 45-year-old British man's quest to lose his virginity to the professional "surrogate sex partner" who inspired the Oscar-nominated film The Sessions.

Clive, an IT worker and devout Christian, travelled to the US to enrol on "radical sex therapy" sessions with Cheryl Cohen-Greene, a surrogate sex partner who charges $300 for two-hour sessions in which she performs multiple sex acts on her client, and finally has intercourse with him.

Cohen-Greene, who makes $50,000 a year, focuses on helping Clive to overcome his fears of intimacy during the documentary, which was screened on Thursday 28 March. Her controversial work, which has been condemned as prostitution in all but name, inspired last year's film, The Sessions, starring Helen Hunt.

"I look at our work like this: If you go to a prostitute, it's like going to a restaurant. You read the menu, you choose what you want, they prepare, they hope that you love it, and hopefully you want to come back," said Cohen-Greene.

"With a surrogate, it's like going to cooking school. You get the ingredients, you learn to make a meal together, and then the point is to go out into the world and share that and not come back."

Around 10 percent of her clients were academics, or career-focused virgins who had neglected their sex lives, she told the New York Post.

"They're like, 'Oh my God, I don't want to come across as inexperienced'," she said. "They finally say, 'Life is too short. I've got to go into surrogate therapy.' "

The documentary, entitled 40-Year-Old Virgins, also features Rosie, a 29-year-old virgin who can't bear to be touched by men, believing they "smell like a cross between aftershave and ham".

"If you want the job done, get an expert," Rosie says of her plan to lose her virginity to Gary, 55, a former tennis professional who now works as a counsellor and sex surrogate.

Channel 4's chief creative officer, Jay Hunt, said of the documentary: "It's heart-warming. I've never seen anything like it.

"They talk in detail about what it's like to have never had any sexual contact with anybody."

Following last year's Oscar-nominated The Sessions, which told the real-life story of the late polio sufferer Mark O'Brien and his experiences with a sex surrogate, Cohen-Greene and other sex surrogates have lifted the lid on details of their controversial profession.

"People tend to be ill-informed about what a surrogate partner does. They think of it pejoratively, the same as a sex worker, but it's not," said Fern Arden, a sex therapist who provides treatment for psychosexual problems at the private clinic she founded in Central Park West, Manhattan.

"Just as you have legitimate massage therapists and people who run massage parlors, there is a huge difference between them.

"The focus is not sex, but familiarity and intimacy. We provide an environment, not for sexual pleasure, but for sexual learning."

Arden's staff of female surrogate partners give clients, mostly virgins in their 40s, one-on-one coaching in "caressing techniques, kissing, feeling relaxed with another person in the nude and, inevitably, the sex act itself", according to the New York Post.

Arden, who has a PhD from the Institute for Advanced Study of Human Sexuality, charges between $3,000 and $5,000 for a course of 12 to 15 sessions with herself and her staff.

There are 30 currently sex surrogates registered in the US under the International Professional Surrogates Association. Surrogates must complete a two-week training program with the society to become registered, and must also be employed by a licensed sex therapist.

The trade operates in a legally grey area, with some believing it amounts to prostitution. Clients "have agreed to pay money for a sexual experience, and everyone understands that's the transaction. In my view, that's prostitution", said Derrelle Janey, a defence attorney at the Manhattan law firm Gottleib and Gordon.

"It doesn't matter if the client is disabled, it doesn't matter if he is suffering from some kind of emotional distress - that just makes it kind of sad."

But Arden defended her practice.

"You would typically need a partner to resolve most sexual problems, and for single men that is obviously an issue," she said.

"Most of the men who come to my centre are sexually inexperienced, so the surrogate programme allows them to progress with their treatment.

"People have this perception of a sex surrogate as, 'Oh wow, I am going to have a sexual teacher and we're going to have hot sex', but it's not that way at all.

"The sessions with the surrogate evolve gradually. It's a very gradual, sensual process of getting used to holding hands, caressing and kissing. Clients could come into treatment for several visits before they even take their clothes off."

Staff and clients at the clinic are tested for STDs every two months.

Sarah, a member of staff with a background in sexual psychology and social work, said: "I usually begin sessions by working on eye contact and the way they hold their bodies.

"We use massage and touch therapy, so the client can learn to be in the moment, be comfortable with their bodies and become aware of the sensations.

"We take a mind-and-body approach and slowly remove anxiety out of the equation."

She refused to disclose whether she had penetrative sex with her clients.

40 Year Old Virgins is available to view on 4OD (UK only).