A vast painting covers the wall above a marble mantelpiece on which rests a human skull. It depicts Maurizio and Roberto Viel, who are twin plastic surgeons, and a nude woman whose breasts they have augmented. The imposing work, called Creation, greets patients at the brothers’ clinic on Harley Street in London. Increasingly these patients are men and, increasingly, they come in search of a larger penis.

“Achieve massive girth”; “Get your giant tool now”: These are genuine subject lines from spam emails that invade our inboxes, offering quack solutions to men who feel small. As the prevalence of porn as well as the increasing exposure of crotches and more on billboards (think David Beckham’s pants ads) helps to fuel a rising sense of inadequacy among many men, penis enlargement has become a growth business.

The problem for those who would make penises larger, however, is that, much more than breasts, for example, they have evolved to withstand great stress and changes in size. They resist almost all attempts to make them permanently bigger.

Roberto Viel says he has overcome these challenges, and performs about 200 penoplasty operations a year. Sitting at his desk, he pulls out a gold-nibbed pen and draws a penis. It includes the suspensory ligament, which holds up the erect penis. By partially severing it, having accessed it by cutting away a flap of flesh covering the pubic bone, Viel causes the penis to drop, and hang lower by as many as two inches. The length of the erection is not increased, however, and its angle is lower. “If I cut too much it will be like that,” Viel says, overlaying his drawing with a drooping outline, “and that’s not the best for sex.”

Viel also extracts fat, usually from the patient’s stomach, and injects it under the skin on the top side of the penis, increasing its circumference by more than an inch, whatever its state. Viel keeps some of the fat in his fridge for top-ups, in case it is reabsorbed. The whole procedure, which takes about 90 minutes and is performed under local anaesthetic, costs around £5,000.

Viel studied medicine in Milan and trained in France and America alongside his brother. They have worked for more than 20 years at their richly-decorated London Centre for Aesthetic Surgery. Maurizio is now based at the clinic’s other branch, in Dubai. In 1991, Roberto gave Maurizio a nose job, which made the twins look more alike. Ten years later, Roberto injected fat from Maurizio’s tummy into his brother’s face, with similar results.

Penis enlargements now make up half the Viels’s work, bringing in a around million pounds a year in revenue. The twins are accomplished self-publicists, and have appeared in dozens of magazines and newspapers. The Independent learned about the growing demand for their procedure in a press release sent by his clinic’s public relations agent. She and Viel shift slightly in their seats when questioning turns to concerns about his work.

Viel estimates he has performed 3,000 penis enlargements since 1991. He is fully licensed to carry out his procedure, inspired in part by the pioneering work in the 1980s of Dr Long Daochao, a Chinese surgeon. But it remains a niche technique. None of the 230 surgeons represented by the British Association of Aesthetic Plastic Surgeons (Baaps), which accounts for about 40 per cent of the industry, offers penoplasty. The Viels are not members. Nigel Mercer, a consultant plastic surgeon and former president of Baaps, says, “It’s not something we say every surgeon should offer because there have been lots of unhappy patients.”

Marcus Drake, a senior lecturer in urology who specialises in reconstruction surgery at the University of Bristol, says, “there is always a reason for a ligament and if you cut it the associated joint or structure wobbles around. A bit of introspection on any man’s part will soon make him recognise this is borderline risky.” Drake goes further, pointing out the lack of full clinical trials of penoplasty. “In the absence of evidence, most people would be sceptical as to the robustness of the procedure and the justification for doing it,” he says. “We’d also regard it as potentially unethical without that justification.”

Viel says the limited availability of penis surgery is not due to risk but because “there is no formal training” for it. He adds: “I have developed and improved the technique using surgical techniques from different surgeries to put together a procedure I think is reliable and gives good results in a safe way. Nobody wants to take any risks, and I would not be so naive or stupid to put my name to a procedure that was not safe.”

Viel also says his success rate is high: “Of course sometimes there are cases when people are not completely happy because of a misunderstanding of expectations before the operation but the likelihood of this is very low.” He also says he offers counselling before and after surgery, and frequently turns away would-be patients. “I say no if they want something I can’t give them,” he explains. “When they want it longer when erect - I can’t do that - or when I think they don’t need it or they have underlying psychological issues.”

Figures collected by Baaps showed 4,300 cosmetic surgical procedures were performed on men last year, a recession-defying rise of 6 per cent. Most popular was the nose job, with 1,000 procedures. Penis enlargement does not feature in the top ten, but Tracey Cox, who writes about relationships and sex, says she’s not surprised that demand is growing.

“I regularly do an online phone clinic for men when they can ask anything about relationships or sex,” she says. “They ask about two things - maintaining an erection, and getting a bigger penis.” Cox blames a growing obsession among both sexes with image, as well as the prevalence of porn. “What men fail to take on board is that men are picked as porn stars for a reason. The danger is that porn is what young boys are using as sex education.”

Viel says his patients vary greatly in age and profession. Steve (not his real name) is 35, works in construction, and lives with his wife and two young children. Four years ago, before Steve was married, Viel injected fat into his penis. Why? “Why not,” he says on the phone from his home in Birmingham. “From a young age I thought it’d be brilliant. I wasn’t worried about my size but I suppose in the end I didn’t want to be Mr Average. I wanted it more for my inner confidence than for showing it.”

Steve only had the fat graft, passing up on the ligament snip (90 per cent of Viel’s patients have both). “When it was done I looked down pretty much straight away,” he recalls. “You’ve got a small incision on your penis with one stitch. I thought, wow, that’s amazing.”

Viel says Steve’s “why not” rationale is unusual and that the majority of his patients, none of whom would talk to The Independent, seek not just a bigger penis, but a better life. “They don’t feel comfortable,” he says. “They don’t want to go to the pool with their son, or to the beach or to play football because they don’t want to change in the changing room. It’s a very sensitive issue.”

Some men become so fixated on size they resort to backstreet surgeons without licences. Baaps warned last year of an alarming rise in silicone implants advertised online for as little as £120. Jim Horton wanted a bigger scrotum. He visited a practitioner at his house and allowed him to inject silicone stored in a milk bottle into his scrotum using “what looked like a sealant gun from a DIY store.” The silicone migrated, causing dramatic hardening and swelling. Horton was too embarrassed to seek help, eventually requiring emergency corrective surgery.

Viel has seen patients who have visited unlicensed surgeons, usually abroad. “The results are unbelievable,” he says. But to what extent does he see himself part of an industry that profits from and fuels paranoia in men who may be vulnerable. “I don’t do that,” he says. “Men are just taking more care about their health and look in the same way more men are going to the gym. Why age in a way we’re not happy with?”