According to The Daily Mail, Cowell has been temporarily replaced by his X Factor colleague Louis Walsh, who joins Amanda Holden and Piers Morgan on the panel.
A source said: "Simon has been terribly sick this week, he has not stopped for months and it has all caught up with him.
"He just needs some rest and he'll be as good as new. He has been under the weather for the past week and it was clear that he could not join the judges today."
They added: "It was all very last minute - Louis only got the call last night, but he's already looking forward to teasing Amanda and Piers mercilessly. You can bet that he will pull no punches."
Britain's Got Talent returns to ITV1 this May.