Gay was given a one-year ban by USADA earlier this month following a positive drugs test for anabolic steroids at the US Championships last year.

He escaped a longer ban by USADA in exchange for what they described as the "significant assistance", a decision which has provoked controversy because no other details were provided.

Among those who have questioned the decision are Sebastian Coe, vice-president of the International Association of Athletics Federations (IAAF). 

Tyson Gay was given a lighter one-year drugs ban after USADA claimed he had provided "significant assistance" to them ©Getty Images

In his defamation lawsuit, Drummond, a member of the US 4x100m relay team that won the Olympic gold medal at Sydney 2000, claims false statements made by Gay to USADA injured his reputation as an honest track and field coach and athlete.

It exposed him to "public hatred, contempt, ridicule and financial injury by portraying him as encouraging the use of performance-enhancing drugs", the lawsuit said. 

According to court documents obtained by the Star-Telegram in Forth Worth, Drummond and Gay visited an Atlanta chiropractor in 2012 and Gay was given supplements to help relieve "nagging injuries".

But the supplements were creams with labels stating that they included banned substances and Drummond claims he sought an explanation warning Gay could not take any "illegitimate" substances.

But Gay later received a shipment of the products from the chiropractor, valued at $9,000 (£5,000/€6,500), which Drummond "unequivocally" recommended that Gay discard as he did not believe they were "safe and appropriate", it is alleged.

At a meeting in Monte Carlo later that year Drummond claims he threw some of the substances away and advised Gay to do the same to the rest.

The pair stopped working together shortly after London 2012, where Gay helped the US to a silver medal behind Jamaica in the 4x100m relay. 

But the following year Gay tested positive and told USADA that the reason was creams Drummond had told him to use. 

Drummond told USADA, according to the lawsuit, that he later remembered a bag of substances that Gay had received in 2012 from an unknown source.

Drummond claims he told Gay not to use anything in the bag, took it from him and stored it under a sink in his home.

He later gave the bag to USADA, he claims. 

But Drummond alleges in the lawsuit discovered that Gay had told USADA that Drummond had injected him with substances from the bag in 2012 and had talked about the sprinter using human growth hormone.

Jon Drummond (left) appeared alongside Carl Lewis, Jackie Joyner-Kersee and Michael Johnson before London 2012 to help launch the Team USA kit ©Nike/Getty Images

"Mr Drummond was a proponent for clean competition when he was an athlete and a coach," said Mark Whitburn, one of Drummond's attorneys, told the Star-Telegram. 

"He categorically denies any wrongdoing."

Drummond, who is now 45 and still an active coach, "was absolutely stunned when rumors began to arise that either Mr Gay himself or others intended to blame this positive test on Mr Drummond," the lawsuit says.

He faces a life ban from coaching if he is found guilty by USADA.

USADA declined to comment.

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