Only 10% of drugs cheats are being caught at the Olympics. former anti-doping chief Dick Pound said.
The London Olympics have been dominated so far by the displays of 16-year-old Chinese swimmer Ye Shiwen, whose world record time in the 400 meter individual medley prompted questions about her performances.
John Leonard, executive director of the World Swimming Coaches Association described Ye's performances as "disturbing" but the young swimmer dismissed the allegations and Olympic officials insist that she is clean until proven otherwise.
"My results come from hard work and training and I would never use any banned drugs," she said.
Pound, who retired from the World Anti-Doping Agency (WADA) in 2008, said "anecdotally, maybe 10% of athletes use drugs and we're catching one or two of them."
He warned that the issue of drug cheating was "getting dumbed down" as sports organizations become "more worried about corruption."
"People who have prepared in advance and used drugs coming here (to London) won't get caught," said Pound.
"If you get caught you fail two tests, a drugs test and an IQ test."
Dick Pound, also a former vice president of the International Olympic Committee (IOC), warned that testers are always playing catch up with the cheaters.
"You'd be pretty ambitious to say London 2012 was a clean Games. There are drugs we don't know about yet so we need to keep finding them and hunting down the distribution networks," he said.
"We know what's going on, but if you're going to do disqualify someone you have to have evidence. We're going to need more resources and more commitment from the sports organizations."
"Basically, they say it is too complicated, too expensive and they're more worried about corruption. It is getting dumbed down."
The IOC announced on Wednesday evening that Uzbek athlete Luiza Galiulina has been excluded from the Games after testing positive for furosemide.
Furosemide is commonly used to stop racehorses suffering nosebleeds, but is banned by WADA because it can be used to mask other drugs.
Galiulina wa provisionally suspended after failing pre-games testing on the 25h July. The 20-year-old's 'B' sample has confirmed the positive result, so the IOC have now expelled the gymnast from the Games.
Galiulina is the second athlete to fail a drugs test at the games after Albanian weightlifter Hysen Pulaku tested positive for the steroid stanozlol, also in pre-games testing.