Her team-mate, Lu Bin, challenged on the second length and also touched inside the old record, clocking 54.15 for the silver. Germany's Franziska van Almsick, the European champion, took the bronze in 54.77, with the Olympic silver medallist, Thompson, out of the medals in fourth place.
Karen Pickering broke the British record, becoming the first British woman to dip under 56 seconds, but it was Le's performance which was met with widespread disbelief around the Foro Italico.
Pickering, who clocked 55.79sec to better her own national mark of 56.11, said: 'I was really shocked that I swam like that and was joint last. I will have to watch it again, but I cannot believe the winner's time. That's a man's time.'
Pickering was the sole British representative on the first night of finals after Nick Gillingham's disappointing failure to make the top eight in the 100m breaststroke. Although he will start as favourite for the 200m on Friday, the City of Birmingham swimmer was expected at least to progress from the heats of the shorter event.
He clocked the ninth fastest time in a tired-looking morning swim in 1:03.02, being more than a third of a second outside the time he registered in winning the Commonwealth silver in Victoria a fortnight ago. The Hungarian Norbert Rozsa went on to win gold in 1:01.24.
China completed a hat- trick of gold medals when Dai Guohong won the women's 400m individual medley and their quartet of Le Ying, Yang Aihua, Zhou Guanbin and Lu captured the 4 x 200m freestyle relay ahead of Germany and the United States.Reuse content