World Anti Doping Agency (WADA) president John Fahey has asked ATP chiefs to "shed light" on allegations made by Andre Agassi that a positive test for banned drugs was swept under the carpet 12 years ago.

Retired professional Agassi, one of only five men to have won all four Grand Slam tournaments, shook the tennis world on Wednesday when he claimed in his autobiography that he used crystal meth in 1997.

Crystal meth is also known as methamphetamine, a stimulant which is on WADA's list of prohibited substances and methods.

After being informed that he had tested positive for the mind-altering drug, Agassi said he wrote a letter to the ATP, the Association of Tennis Professionals, claiming he had taken it by accident and asked for leniency.

No further action was taken.

Although affirming that no retroactive action can be taken against Agassi, Fahey said he hoped the American's "disappointing" admission would be used in a positive way to help educate youth about the dangers of drugs.

The WADA chief however appeared less understanding with the ATP, which oversees the men's professional game.

"We would hope that Andre Agassi might now see his way to be a role model and alert youth and tennis players to the dangers of drug use and doping," said Fahey.

He said: "The World Anti-Doping Code, which took effect in 2004, has an eight-year statute of limitations and it is unlikely that any action may be taken by tennis authorities against the player, if methamphetamine was banned under the ATP list of prohibited substances and methods at that time.

"WADA would however expect the ATP, which administered its own anti-doping program at that time, to shed light on this allegation."

In the book, Agassi said after being informed of his positive tests he wrote a letter to the ATP was "filled with lies interwoven with bits of truth". The ATP reviewed the case - and threw it out, the newspaper cites the book as saying.

In a statement issued on Wednesday, the ATP said: "Under the tennis anti-doping program it is, and has always been, an independent panel that makes a decision on whether a doping violation has been found.

"The ATP has always followed this rule and no executive at the ATP has therefore had the authority or ability to decide the outcome of an anti-doping matter."

The International Tennis Federation (ITF) meanwhile claimed tennis runs one of the most rigorous dope testing procedures in professional sport.

"The ITF is surprised and disappointed by the remarks made by Andre Agassi in his biography admitting substance abuse in 1997," said the statement.

"Such comments in no way reflect the fact that the Tennis Anti-Doping Program is regarded as one of the most rigorous and comprehensive anti-doping programs in sport.

"The events in question occurred before the World Anti-Doping Agency (WADA) was founded in 1999 and during the formative years of anti-doping in tennis when the program was managed by individual governing bodies.

"The ITF first signed the WADA Code in 2004 and the Tennis Anti-Doping Program undergoes constant review and improvement."

It also said: "The ITF, Grand Slams, ATP and WTA Tour are now unified in their efforts to keep tennis free of drug use and this should not be overshadowed by an incident that took place more than 12 years ago.

"The statements by Mr Agassi do, however, provide confirmation that a tough Anti-Doping Program is needed."