O'Sullivan admits in her autobiography, Sonia, My Story, to be published on Thursday that she did not know when she raced who was on drugs and who was not.

In the book, produced with the help of the Irish Times' award winning sportswriter Tom Humphries, O'Sullivan writes: "The cheats?

 "Are they laughing at us?

 "I supposed they must be.

 "You can't think much about it.

 "You have to close it off and get on with it.

"People make the point to me that, from Barcelona [the 1992 Olympics] onwards, they can find cheats in my races; when they subtract them from the equation, I should have got this and that medal.

 "Even Gabriela Szabo had her controversy.

 "In 2003 a car belonging to her was pulled in and found to have EPO (Erythropoietin) in the boot.

 "She denied all knowledge, but her career was a bit up-and-down after that.

"It's not for me to wonder any more.

"I never really thought of Gabriela Szabo in that way because she never ran or achieved anything that I believed I couldn't match.

"She had a couple of dominant years, and I had the same. 

"But did she do things she shouldn't have been able to?

 "I don't know."

Szabo beat O'Sullivan in Sydney in one of the closest races in Olympic history to claim the gold medal by less than quarter-of-a-second on a night of such fantastic athletics that is generally considered to be the greatest in the history of the sport.

"Magic Monday", as it was dubbed, also saw Australia's Cathy Freeman win the 400m gold medal to fulfill the hopes of a nation before a capacity crowd of 112,000 in the Olympic Stadium, Ethiopia's Haile Gebrselassie beat Kenya's Paul Tergat in a memorable 10,000m and Britain's Kelly Holmes take a surprise bronze medal in an 800m won by Mozambique's Maria Mutola.

The incident that O'Sullivan refers to in her book came when the French authorities discovered an undeclared substance in the boot of Szabo's car, driven by a family friend, for delivery to teammate Mihaela Botezan.

A week later they cleared Szabo of any charges.

She retired from athletics in 2005 amid health problems and a decline in her performance, citing exhaustion.

Szabo is now the vice-president of the Romanian Athletics Federation.

O'Sullivan, the 1995 world champion and Ireland's greatest-ever female athlete, had a long rivalry with Szabo throughout her career.

Those Sydney Games will also be remembered for the five medals, including three gold, won by American Marion Jones but which have subsequently been taken away from her after she admitted  being on performance-enhancing drugs at the time. 

O'Sullivan writes in her book: "[She] has been a disaster for our sport.

"In a way I was surprised she was caught.

 "I really wouldn't have thought she was cheating.

"That is how innocent I am about the thing.

 "To me, she was so much better and she had the shape, bigger and quicker, so of course I thought she would win because she is built to win it."

Sonia, My Story is published by Penguin Ireland and will be available from October 2