World Anti-Doping Agency president John Fahey has urged drug cheats to withdraw from the Olympics and Paralympics to keep this summer's Games clean.
Fahey said all athletes have the right to compete on a clean playing field and that no cheater could ever consider themselves a winner.
"I say this in the clearest way possible: if you are a doping athlete and you are planning to compete in London then you must withdraw from your Olympic team," said Fahey.
"Doping is cheating, plain and simple. And if you compete in London as a doped athlete then not only will you be cheating your fellow athletes, you will be cheating sports fans across the world, doing a disservice to your national flag and flouting the ideals of the Olympic Movement.
"A doping athlete cannot achieve success, it is a complete contradiction. Even if a doping athlete were to win a medal he or she would never be able to look at themselves in the mirror and say, 'Well done, I deserved this'.
"The Olympic Games is the absolute pinnacle for many athletes, and for them to train endlessly over a four-year period and then have their efforts belittled by a doping athlete, to me that is complete and utter betrayal of what sport stands for."
Fahey added that he was confident that drugs cheats would be caught and punished thanks to the efforts of the International Olympic Committee and the Games organisers, who plan to conduct up to 6,250 tests.
"There has been a coherent effort to make London 2012 as 'clean' as possible and doping athletes should know that their chances of avoiding detection are the smallest they have ever been," he said.
But while he is confident that dopers will be caught, Fahey said it was up to those who know they are cheating to do the honest thing and not deny fellow competitors their just rewards on the night.
"Athletes are responsible for what they put into their bodies and athletes are responsible for whether or not they choose to dope," he said.
"The world's ant-doping community can only do so much. If every athlete decides not to dope then we will have a completely dope-free Games, that's the simple reality.
"It is up to the athletes and I urge them to collectively take more responsibility for the sake of clean competition."