Michael Johnson believes American sprinter Tim Montgomery must be allowed to compete at the Olympics.
Johnson believes the case is highly complex
Montgomery, the 100m world-record holder, is currently being investigated for possible doping violations.
But five-time Olympic champion Johnson told BBC Radio Five Live: "At this point the rules say he can compete because he has not been found guilty.
"And he should be allowed to because, if results find him not guilty, he will have unfairly missed out on competing."
Montgomery's case is to be heard at the Court of Arbitration for Sport, which has been asked by the US Anti-Doping Agency to consider a lifetime ban.
The 100m world record holder denies taking performance-enhancing drugs and has never failed a drugs test.
Johnson said the case would be difficult, adding: "It will not be easy to find an answer without a positive test".
"That's why it is so complicated to get to the bottom of this," he said.