"The pictures were taken while I was on holiday by someone who, in an act of cowardice, has shot without permission and then used the pictures for commercial gain," Kaif, 30, wrote in the letter.
The actress said the photographs, where Katrina is seen in a red and white bikini with Ranbir at a beach in Spain, were an invasion of her privacy.
"I request that all media running these pictures please refrain from doing so. I have a wonderful relationship with the media and have been accessible to the media at all times. There is no reason for this furtive and invasive behaviour," she added.