Sienna Miller developed her sense of style as a "visceral reaction" to being forced to dress the same as her stepsister

Sienna told Grazia magazine: "I inherited a stepsister, and his new wife used to put us in identical clothes - I had this visceral reaction to it all and started going quite off piste with my fashion choices.

"I'm only realising the impact it had as I talk through it now.

"It became important for me to assert my identity - I'd wear a red shoe with a green shoe, I was pretty wacky with my styling back then...

"I was quite out there and individual, but in a grungy, naughty, smoking fags sort of way. I wasn't like Regina George [from Mean Girls]."

While Sienna is the face of Schwarzkopf Crème Supreme and has been praised for her glossy tresses, the Anatomy of a Scandal actress was horrified when boyfriend Oli Green - with whom she is pregnant with their second child together, having given birth to their daughter in 2024 - compared her hair to a horse recently.

She said: "It's a bit horsey.

"My boyfriend likened it to a horse tail the other day - I think he meant it was thick, you know, but I thought, 'Great, excuse me while I go and do a hair mask.' "

Sienna is midway through her pregnancy and gearing up to lose some of her hair again after she has given birth.

She said: "It's these bits. They're just starting to grow back properly and now it's all going to go again."

The Factory Girl star, who also has 13-year-old Marlowe from her relationship with Tom Sturridge - admitted she "aspires" to lead a clean wellness-first lifestyle but is realistic enough to know she wouldn't stick to such a tight regime.

She laughed: "That's the person I aspire to be. But if I have a 5am pick-up, I'm not going to get up at 4.15 to do a workout and stir collagen into a cup of tea. I'm going to set my alarm for 4.55, brush my teeth and go."