The actress refused the role in the Karan Johan-produced movie because of a scheduling clash, Mumbai Mirror reports.
"Katrina Kaif had to opt out of the film due to prior commitments," her spokesperson told the paper. "Katrina was very keen on working with Karan Johar but couldn't rework the dates to allot for the film, thanks to schedules already set in place."
The Tees Maar Khan actress added: "I didn't have dates and thus couldn't do Ayaan's film. I prefer doing not more than two films in a year so that I can focus on those films completely."
Kaif dated Kapoor after they co-starred in Rajneeti following her split from on-off boyfriend Salman Khan.
The couple ended their relationship of nearly 18 months after Kapoor was rumoured to be seeing model-turned-actress Nargis Fakhri, who appears with him in Rockstar.
Kaif's decision to drop out of Johar's upcoming project ends media speculation over the casting of the female lead.
Johar had also reportedly considered Kapoor's ex-girlfriend Deepika Padukone for the role.
Watch Katrina Kaif in Tees Maar Khan below: