McIntosh clocked eight minutes 11.39 seconds at a sectionals meeting in Orlando, Florida on Thursday, nearly six seconds faster than Ledecky.

Ledecky suffered her first defeat in the 800 since 2010, while McIntosh's swim made her the second fastest woman in the distance behind the American world record holder.

Ledecky still owns the 16 fastest times in history, including her 8:04.79 world record.

The American has dominated the 800 like no other, winning successive gold medals at the London, Rio and Tokyo Olympics.

She won her sixth consecutive world title in the 800 at last year's championships in Fukuoka.

She is expected to defend her 800 gold at Paris and bid to become the first woman to win four successive titles in Olympic swimming.

McIntosh has rapidly established herself as one of the world's top swimmers, winning back-to-back world titles in the 200m butterfly and 400m medley.

She has focused on mid-distance events since missing the 800 final at Tokyo as a 14-year-old but the result in Florida may encourage her to return the 800 to her Olympic programme.

Both McIntosh and Ledecky have elected to skip the World Championships which start in Doha on Sunday but Ledecky's upset was a hot topic among the entrants in Qatar.

"I have some friends of the Canadian team and I know Summer pretty well," said American Hunter Armstrong, who will defend his 50m backstroke world title in Doha.

"I'll have to shoot her a text and congratulate her," he told reporters on Friday.