Bolt, who celebrated his 22nd birthday on Thursday, clocked 19.30sec to complete the first Olympic sprint double in 24 years and become the first athlete in history to also smash the world mark in both events at the same Games.

"This guy is Superman II," Johnson told the BBC. "When I saw the start he made to the race, I just thought 'wow'. It was a much better start than the one he made in winning the 100m on Saturday. This is Bolt's favourite event and he wanted that record; he went for it, he came here focused on it. He knew that most likely he'd win gold but gave it every single thing he had."

Johnson had believed that Bolt was unlikely to beat his 1996 world record here, but he said he was stunned to see the amount of work he had put in pay such rich dividends.

"My concern, and the reason I thought he might not get the world record here, was that he wouldn't have been working on his speed endurance, the ability to hold that speed for the entire race," said the American who still holds the world record in the 400m.

"I don't see any reason why he couldn't now move up to the 400m but I'm not sure if he'll be able to break the world record in that.