Cowell offered the former Strictly Come Dancing judge an invite to irritate the BBC, The Sun reports.
"He just couldn't resist inviting her to his birthday," a source said. "It's a gesture aimed squarely at the BBC."
Cowell is allegedly "still fuming" that the BBC refused to move Strictly Come Dancing to avoid a clash with The X Factor.
Among the guests expected to attend Cowell's party tonight are Sir Elton John, Kylie Minogue and Donald Trump.