Cowell met Ana Paula Junqueira, who was once married to tennis equipment tycoon Johan Eliasch, in January in the Caribbean, the newspaper said.
Tom Bower wrote in his book, "Sweet Revenge, The Intimate Life of Simon Cowell," that Cowell invited Junqueira on board his yacht Slipstream along with some of her friends.
"Fuelled by champagne he invited a group of Brazilian girls on to Slipstream. By 5 a.m. he was intimately embraced with Ana Paula, a well-known international socialite," Bower wrote.
"At teatime later that day, she had flown back to Sao Paulo on a private jet and had texted her intentions. 'She's coming to London soon,' he announced. 'That should be fun,' Cowell smiled," the book said.