Anti-doping laws could stop Jarryd Hayne from competing at the Rio Olympics, according to former ASADA boss Richard Ings.
Hayne quit the NFL on Monday morning to attempt to earn selection with Fiji in rugby sevens at the Games.
However Ings said World Rugby rules require players to be in a registered testing pool for six months to be eligible to compete at sanctioned competitions.
And according to the former ASADA chief, Hayne's stint in the NFL does not fit the requirements.
"If Jarryd Hayne had been playing in WADA compliant sport he would have grounds to reduce the 6 months," Ings tweeted on Monday morning.
"But NFL is PED badlands. No chance."
Ultimately, the decision on whether Hayne would be cleared to play will come down to the International Rugby Board, while the former NRL star could appeal any decision which goes against him.
But Ings believes the restriction is a vital one.
"The six-month return to competition rule is standard across most sports for athletes returning to international competition. And necessary," Ings tweeted
"The rule equally applies to reinstatement or to new players. Sensibly it must or any NFL player could bulk up and play."
Hayne's hopes of being cleared to play should increase though given fellow NFL-convert Nate Ebner debuted for the United States at last month's Singapore World Sevens tournament - just a month after he left the New England Patriots.
Ings, who was chief of ASADA for six years, also indicated similar rules apply to the NRL.