Actress Ella Purnell has opened up about her teenage battle with anxiety and depression — saying she wants to share her experience to help others.

The Londoner, 21, currently on screen in the BBC’s adaptation of Agatha Christie’s Ordeal By Innocence, juggled her education with films such as Never Let Me Go, Maleficent and Kick Ass 2.

In an interview with ES Magazine, she described how an incident at school affected her. “I have anxiety and have had depression in the past,” she said.

“When I was 15 or 16 I had a bad experience at school — some issues with other classmates and somebody I was seeing, we don’t need to go into it. I started self-harming and did that for a long time until my mum caught me and I started to learn about it [mental health].

“I thought depression was something you could only have if your parents died, or you were really ill and about to jump off the bridge. I certainly didn’t feel comfortable enough to talk about it then.”

She added: “Self-harm is not about attention — I hid [my scars] for six months, wore long T-shirts, covered them up with make-up. It’s about punishing yourself, for people who dislike themselves or suffer from insecurity or self-doubt.

“They want to hurt themselves because they think they deserve it: that’s where I was at. I had to work very hard to remove my trigger points and to create a safe space so I wouldn’t become tempted to get back into it again.

"I still have the scars and it’s something that I did that I will always regret: it hurt a lot of people around me and it’s on my body forever.”

Purnell, who now wants to work with a mental health charity to raise awareness, made her name playing younger versions of stars including Keira Knightley, in 2010 film Never Let Me Go.

She describes the actress, 33, as “ one of my mentors” and said she “will turn to her for industry advice or reassurance or validation at any time”.

Now a star in her own right, Purnell takes the lead role in TV series Sweetbitter, based on Stephanie Danler’s novel about a New York waitress.

She said: “It was the first time I had done nudity. I was terrified and it was awkward as hell, but the most important thing for me was that it was real, that it doesn’t look like a porno, there’s no shaved things and no weird uncomfortable sexual positions, and it’s sometimes awkward and gross.”

She said she had never experienced sexual harassment herself but it was “good to have these conversations” about the #MeToo movement.

On the power dynamics of the industry, she said: “It’s very hard to not give in, to sell your soul: to flatter a more experienced male person of power, a producer or director, in order to get what you want.

"You feel a pressure to flutter your eyelashes and flirt, because you know that will work, and I think it’s admirable to not do that. I don’t think that is a comment on women, I think it is a comment on men in power.”

For confidential support on mental health call the Samaritans on 116 123, email jo@samaritans.org or attend a local Samaritans branch.

Full interview in ES Magazine – free every Thursday and Friday