The International Association of Athletics Federations (IAAF) will help the U.S. Anti-Doping Agency (USADA) defend an appeal by Olympic 100 metres champion Justin Gatlin against a four-year doping ban.

"We will be part of the defence in the Court of Arbitration for Sport," IAAF spokesman Nick Davies said on Friday. "We are going to support USADA."

A U.S. arbitration panel suspended Gatlin for four years after a positive test for the male sex hormone testosterone in 2006.

The panel ruled that the positive was a second offence after Gatlin tested positive in 2001 for medication to treat Attention Deficit Disorder. He was reinstated by the IAAF in 2002.

Gatlin has appealed to CAS and in a statement on Thursday his attorney Maurice Suh said he wanted a ruling by the end of May.

Gatlin's lawyers argue that if the panel had not considered the 2001 test in its verdict he would probably have received a two-year suspension as a first-time offender.

A two-year ban would have made eligible to return to competition in May, allowing him to run in June in the U.S. trials for the Beijing Olympics.