World Anti-Doping Agency (WADA) president Craig Reedie can understand why athletes who had confidential medical data hacked would be "irritated" and knows the breach "casts doubt on the anti-doping system".

WADA revealed on Tuesday that it was hacked by a Russian cyber espionage group going by the name of Tsar Team (APT28), also known as Fancy Bear.

Gymnast Simone Biles, winner of four gold medals in the Rio Olympics last month, Serena and Venus Williams were among the athletes whose information was accessed by the group.

There is no evidence of any wrongdoing by the athletes involved, with the hackers having disclosed instances where therapeutic use exemptions (TUEs) have been granted, enabling competitors to use medications for legitimate reasons. 

Venus Williams expressed her disappointment that her data was published without permission, while Biles revealed that she had been taking medicine since she was a child because she suffers from attention deficit hyperactivity disorder.

Reedie knows such private information should not have been accessed and said there can be no repeat.

He told Omnisport: "The therapeutic use exemptions have been in place for some time and it works well. They [athletes] are quite right to be irritated that this has happened, we are irritated that it has happened as well and it shouldn't have happened. 

"Clearly it is unacceptable and we must do everything we can to make sure it doesn't happen again."

He added: "This is unfortunate in the extreme because it casts doubt on the anti-doping system, which has worked pretty responsibly and pretty well over the previous 17 years.

"There is also a further complication in that we understand that the people involved in hacking have connections in Russia, which isn't particularly helpful when we are currently involved in trying to make the Russian anti-doping system compliant with the World Anti-Doping code.

"There are questions over whether that process will be any easier."