Johnson and Pettigrew celebrate their gold medal in Sydney

Five-times Olympic champion Michael Johnson says he will return the relay gold he won at the Sydney Games after a former team-mate admitted taking drugs.

Antonio Pettigrew, who together with Johnson won 4x400m relay for the United States at the Sydney Olympics in 2000, has admitted taking banned substances.

"The medal was won unfairly. Relay is a team effort and one member cheated," Johnson told BBC Radio Five Live.

"I am very disappointed. I had considered him a friend."

Pettigrew made his admission last month at the trial of Trevor Graham, coach of disgraced sprint trio Justin Gatlin, Marion Jones and Tim Montgomery.

The 40-year-old testified that Graham encouraged him to inject human growth hormone and EPO, both banned substances, from 1997.

I have moved the gold from the location where I have always kept my medals because it doesn't belong there, and it doesn't belong to me Michael Johnson

His confession follows drugs bans on Alvin and Calvin Harrison as well as Jerome Young who were also members of the victorious US team.

Young, who ran in the heats but did not compete in the Sydney final, failed a drugs test prior to the Games but was allowed to compete because the American authorities had chosen not to take any action against him.

Calvin Harrison tested positive for a banned stimulant in 2003 and was suspended for two years while Alvin accepted a four-year ban in 2004 after admitting he using performance enhancers.

The 4x400m relay victory in 2000 was the fifth and final Olympic gold of Johnson's illustrious career.

The 40-year-old also won nine World Championship golds and still holds the world record in the 200m and 400m.

"There are some people in sport you become friends with and, while we competed against each other, we developed a friendship early on," added Johnson.

"So it was very disappointing to hear he had been using banned substances.

"I liken drug use to crime in society. People don't go and tell their best friends [what they have done].

"The Olympics is the pinnacle and what you strive for and work so hard to be a part of. That medal doesn't belong to me any more.

"This has caused a lot of pain. The other teams that finished behind us will wonder whether they could have won and, I believe, we could have done so with other American athletes - clean athletes."