Sienna Miller thinks Keira Knightley will be a ''perfect'' mother.

The 'American Sniper' actress - who has two-year-old daughter Marlowe with partner Tom Sturridge - insists she won't be offering any advice to her friend, who is expecting her first child with husband James Righton, because she will be an ''amazing'' parent and doesn't need help.

She told the New York Post newspaper's Page Six column: ''We've spoken a lot and I know she will be amazing.

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

Sienna, 32, 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.''

Though Sienna insists her life with Tom, 29, and Marlowe is very ''normal'', she is adamant their day-to-day domesticity isn't boring.

She said: ''It's very normal, very kind of still.

''We go to the country. I like to cook and go to Whole Foods. I have a [Newfoundland[ dog - not a dog but a bear - who when I walk pulls me over flat on my face into the mud.

''My baby pukes on me. It's life. It's very much a normal life.''

''I really don't want to sound boring. It's completely crazy and illicit behind closed doors.''