MacNeil delivered a stunning burst of speed over the final 50m to touch in 55.59s and edge China’s Zhang Yufei (55.64s) into second while Australia’s Emma McKeon (55.72s) who took bronze.

Sjostrom finished seventh, well off the pace.

“I actually have no words right now, I can’t believe it,” said MacNeil, the world champion who now has her first Olympic gold medal.

“I couldn’t see anyone. I was in the outside lane which I think worked to my advantage. I could focus on my race rather than looking at what other people were doing.”

Zhang turned in front at the half-way mark followed by American Torri Huske, only for MacNeil to blast past them to hit the wall fractionally in front.

Sjostrom, who broke her elbow earlier this year and said the butterfly was her lowest priority in Tokyo, holds the world record of 55.48s set at the Rio Olympics.