US runner Tyson Gay crosses the finish line in the men's 100 meter dash during the US Olympic Athletic Trials in Eugene, Oregon in this file photo taken June 24, 2012. [Photo/Agencies]

RALEIGH, North Carolina - Former world double sprint champion Tyson Gay has been suspended for one year after testing positive in 2013 for a banned anabolic steroid, the US Anti-Doping Agency (USADA) said on Friday.

Gay has also returned the silver medal he won with the US 4x100 meters relay team at the 2012 London Olympics but will be eligible to return to competition as early as next month after admitting his offence and co-operating with investigators.

"We are thankful he decided to come in and be truthful in his cooperation with us," USADA chief executive Travis Tygart told Reuters.

"With the loss of results since 2012, including an Olympic medal, he has suffered serious consequences, but by cooperating he has tried to help the sport move forward for the good of clean athletes."

As part of the suspension he accepted, Gay, the joint second fastest man in history, was disqualified from all competitive results since July 15, 2012, the date he first used a product that contained a prohibited steroid.

The start of his ban was backdated to June 23, 2013, the day his doping sample was collected at the USA Outdoor Track & Field Championships and when he voluntarily stopped competing after being notified of his positive test.

His first race back is planned for July, Reuters has learned.

The American 100 metres record holder had tearfully admitted last July he had failed an out-of-competition doping test, saying "I basically put my trust in someone and I was let down."

"I don't have a sabotage story. I don't have lies," Gay, who had previously spoken out against doping, said in a 2013 interview, adding he had never knowingly taken a performance-enhancing drug. "I made a mistake."

The 31-year-old could have been suspended for two years for his first doping offence but received a reduction because of his substantial assistance, USADA said.

The circumstances of his case is still under investigation and could produce additional suspensions, track and field sources said.

The ban is also subject to appeal by the International Association of Athletics Federation and the World Anti-Doping Agency.

The United States Olympic Committee (USOC) confirmed it had received the medal Gay won at the London Olympics and would be forwarding it to the International Olympic Committee (IOC).

"We were extremely disappointed to learn that Tyson Gay committed a doping offense," USOC spokesman USOC spokesman Patrick Sandusky said in an email.

"The USOC remains committed to the fight against doping and will continue to invest substantial resources to ensure that the United States is and remains a world leader in anti-doping efforts."

The IOC will determine whether other members of the US relay team will be allowed to keep their medals. The committee has offered varying opinions in past cases.

Whether the United States record the team of Trell Kimmons, Justin Gatlin, Gay and Ryan Bailey set in finishing second to Jamaican in the 4x100 relay stands also will be determined by the finalization of results from the race, USATF spokeswoman Jill Geer said.

The governing body expressed disappointment that one of its biggest names had tested positive.

"USA Track & Field is gravely disappointed any time an athlete uses performance-enhancing drugs, and Tyson Gay's case serves as a lesson about the consequences of making poor decisions," USATF chief executive Max Siegel said in a statement.

"We appreciate that Tyson accepted responsibility and has assisted USADA by providing information to help battle the use of PEDs. We thank USADA for their vigilant work on this case and for their leadership in the pursuit of clean sport."

A spokesman for Gay said his client was unlikely to make any public comments on the case for several days.