A gruelling regime of training and dieting has paid off for an Oxfordshire bodybuilder who flexed his muscles on a national stage to become Mr Britain Junior at the first attempt.

Rob Waterhouse, 22, scooped the top award in his age category at the contest in Glasgow on Sunday in his first-ever finals appearance.

Stringent anti-doping rules imposed by the British Natural Bodybuilding Federation mean each competitor is given numerous drugs test and is strapped to a lie detector to make sure they have been clean for at least 10 years.

The personal trainer from Abingdon was joined by his boss Darren Loxton, from Didcot, who finished fifth in the lightweight category.

Mr Waterhouse, who only started training seriously 14 months ago, said: "I'm absolutely delighted. To win this in my first year is just amazing."

The former John Mason School pupil lost more than three stones before the final and reduced his body fat from 24 per cent to three per cent.

He said: "I took part in a qualifier in June and was dieting from March until June for that. From then I went back on my normal diet and was bulking back up until late August when I started dieting again.

"It wasn't always pleasant. Weekends are a struggle when you're out of your routine and your friends are all out drinking. I just had to live mainly on chicken, brown rice and protein shakes, eating every two and a half hours."

Mr Waterhouse, who decided to compete after a suggestion from someone at his gym, admitted bodybuilding had a reputation for steroid use, but added: "I wanted to stay away from the drugs and the stereotype that goes with it — people think it's just the drugs that get bodybuilders into that shape, but what we do is totally natural."

Mr Waterhouse, who weighed just over 12 stone for the competition, performed a series of eight compulsory poses and a 90-second freestyle routine in front of the judges.

He used fake tan and went without water for 30 hours prior to the event to show off his definition.

"Before you go on stage you have a brandy to bring out your veins," he said.

"It also gives you a bit of Dutch courage. I was confident going into it, but there was a guy who came second in the world championship last year and he was also the reigning British champion.

"I knew it would be tough, but in the end he came second behind me."

Mr Waterhouse was today "pigging out on chocolate and sweets" but may start dieting again today if he gets the call up to the world championships in Washington DC in November.

Mr Loxton, 33, the manager of Feel Good Fitness in Abingdon, said: "People associate bodybuilding with steroids and think there is no way you can do this naturally, so it's a fantastic achievement.

"For my first year of competing finishing fifth was fantastic."