It comes after information about the bash appeared in the press earlier in the week, including a copy of his e-invite.
A source told The Sun on Sunday: “No stone is being left unturned; Simon and his team are on a mission.
“Inspired by Coleen and her intrepid Wagatha Christie efforts, Simon is wading through his party list, trying to figure out possible motives: greed, publicity, desperation or revenge.
“The invites were all sent out from a secure email address, so this is being looked at, too."
The X Factor judge was due to have a dinner at La Familglia restaurant in London but postponed it due to work commitments.
Yahoo News UK has contacted representatives of Cowell for comment.
In October, Rooney, 33, made headlines when she claimed stories about her were being sold to tabloid newspapers from fake information in Instagram posts which she had made visible to fellow WAG Rebekah Vardy's account only.
This has been a burden in my life for a few years now and finally I have got to the bottom of it...... pic.twitter.com/0YqJAoXuK1— Coleen Rooney (@ColeenRoo) October 9, 2019
Vardy, 37, has vehemently denied any wrongdoing, saying she isn’t the only one to have had access to her Instagram account.
In her response to Rooney's post, Vardy shared: "If you thought this was happening you could have told me and I could have changed my passwords to see if it stopped.
"Over the years various people have had access to my Insta & just this week I found I was following people I didn’t know and have never followed myself."
The war of words between the two women went viral, with the situation even spawning Halloween costumes.