LONDON: Michael Phelps may be the most successful athlete in Olympic history but he is "probably not" the greatest, according to London Games chief Sebastian Coe.

American superstar Phelps powered into the record books in the swimming pool Tuesday, bagging his 18th and 19th medals to become the most decorated Olympic athlete of all time.

But while Phelps' astounding medal tally, which includes 15 golds, saw many pundits rush to anoint him as the greatest of all-time, Coe questioned whether the Baltimore Bullet deserved that accolade.

"I think you can clearly say that by the medal tally he is the most successful. My personal view is that I'm not sure he's the greatest.

"But he's certainly the most successful," said Coe, himself a two-time Olympic gold medalist.

Coe however declined to be drawn on who he thought might deserve the tag of greatest Olympian, although he suggested that cases could be made for both Jesse Owens -- a four-time gold medalist at the 1936 Berlin Games -- and Carl Lewis, who won golds at four consecutive games between 1984 and 1996 as well as winning four at a single Olympics in 1984.

"In the lead-up to these Games we've had the great British pub game of who is the best British Olympian when we were talking about who would be lighting the cauldron," Coe said.

"This is the global pub game -- who is the greatest Olympic athlete of all time? It's self-evident that the guy's won how many medals? 19? It's a pretty good haul. But whether he's the greatest I don't know.

"I could go around this whole room and come up with different interpretations. You have to say he's up there, but whether he's the greatest? In my opinion, probably not."