Factory Girl actress Sienna Miller won't be giving her pal Keira Knightley parenting advice because she think she will be a "perfect" mother.

The 32-year-old star, who has two-year-old daughter Marlowe with partner Tom Sturridge, insists that she doesn't need to offer Knightley, who is expecting her first child with husband James Righton, any words of parenting wisdom.

Speaking to New York Post newspaper's Page Six column, Miller said, "We've spoken a lot and I know she will be amazing.

"She doesn't need any advice. She will be perfect."

Miller recently revealed she was "overwhelmed" by how natural it felt when she became a mother.

She said: "I was overwhelmed by how normal it felt. It was like, 'There you are - that's what I have been missing.' Like we're both in on something only we know. An amazing sense of being complicit with a little being."