The actress is to play a gymnast opposite Aamir Khan in the action thriller.
Personal trainer Yasmin Karachiwala told The Times of India: "Kat needs to get flexible and lean, so when she returns from London, we will change her workout routine.
"She needs to do lots of Pilates to open up her muscles and increase flexibility, apart from a combination of functional training and TRX to develop core strength. She doesn't need to look thin, but lean."
In September 2011, Kaif reportedly insisted on a no-bikini clause for the third film in the franchise.
It was reported last week that streets in West London had been closed off during the filming of key stunts in Kaif's currently untitled Yash Raj project.