Kate Hudson has called Kurt Russell 'dad' for years, but her biological father exclusively tells RadarOnline.com it's "really painful" to hear his daughter refer to another man as her dad.

"At first it was okay when everybody was getting along, but there were certain things I didn't like. I didn't like that Goldie wanted the kids to call Kurt 'pa,'" Kate's father Bill Hudson told us in an exclusive interview.

"It was very confusing to the kids, particularly Kate, and that was really painful."