Justin Gatlin beating Usain Bolt in the Jamaican sprint star's final 100 metres outing at the IAAF World Championships on Saturday was "not the perfect script", admits Sebastian Coe.

Bolt was denied a historic fourth Worlds gold in the blue-riband event by Gatlin, who has served two doping bans and was roundly booed after denying his long-time rival a last slice of individual glory.

IAAF president Coe confessed it was not the result he desired with Bolt leaving the sport after next weekend's 4x100m relay, but stated that Gatlin is ultimately eligible to compete.

"It's not the perfect script. I thought Usain was very generous with the observations he made," Coe said on BBC Radio 5 Live.

"That must have been a bitter event for him to swallow. He was bigger than the moment and it typifies his career.

"I'm not eulogistic that someone who has served two bans has walked off with one of our glittering prizes.

"But he is eligible to be here."

After being granted early reinstatement following a failed drugs test in 2001, Gatlin was handed an eight-year suspension for a second adverse finding in 2006, reduced to four on appeal.

Coe is still open to the use of lifetime bans for athletes found guilty of doping, but believes legal issues will continue to thwart their implementation.

"There have been two bans in the past [for Gatlin], one which got watered down which made it very difficult for the second ban," Coe said.

"The second ban we went for an eight-year ban which would have in essence been a life ban - we lost that. So these things are suffused in legality.

"I'm not going to close the door on lifetime bans but we've constantly tried it and lost it in a mixture of courts and particularly the court of arbitration [for sport].

"It's worth remembering that Gatlin's first ban was for amphetamines, and the case against him was at the more serious end. That then got watered down. We then had the second major infringement. We applied for an eight-year ban and again that got lost."

Gatlin has been booed throughout London 2017 and Coe is unsure if that will continue during Saturday's medal ceremony, but he hopes it does not overshadow what could be Bolt's last appearance on the podium.

"I don't know what will happen but we have to remember that we're saying goodbye to an athlete who has done an extraordinary amount for the sport," he said.

"It's not the most exciting day in prospect for me but he has to be accorded some respect."