The British supermodel - who has been friends with Simon for several years - plans to serenade the music mogul with a surprise performance at his star-studded 50th birthday bash on October 7.
However, the 35-year-old catwalk beauty is unsure about what to sing.
A source told Britain's Daily Star newspaper: "It's a toss up between a sexy Marilyn Monroe-style 'Happy Birthday', or her speciality Lily Allen's 'Not Fair', which she has been practising with her friend Lily."
Simon has invited 400 celebrity friends to help him celebrate reaching the landmark age at the black tie bash at Hertfordshire's Wrotham Park.
Guests expected to attend include music legend Sir Elton John and his partner David Furnish, Hollywood actor Kevin Spacey, actress Denise Richards, Australian pop star Kylie Minogue, the judges from his talent shows and most of his recording artists.
Kate will have to ensure her singing is pitch-perfect on the night as she will be performing alongside some of the world's biggest acts, including Leona Lewis.