Cillian Murphy ate an “almond a day” to get his gaunt J Robert Oppenheimer look.

The ‘Peaky Blinders’ actor, 47, has refused to say how he lost weight for the part as the father of the atomic bomb in Christopher Nolan’s ‘Oppenheimer’ film, but his on-screen wife Emily Blunt, 40, has now revealed how he made his transformation.

Blunt, who plays Kitty Oppenheimer, told Extra: He had such a monumental undertaking. And he could only eat, like, an almond every day.

“He was so emaciated.”

Murphy told The New York Times in May: “I love acting with my body, and Oppenheimer had a very distinct physicality and silhouette, which I wanted to get right.

“I had to lose quite a bit of weight, and we worked with the costume and tailoring; he was very slim, almost emaciated, existed on Martinis and cigarettes.”

But the actor told in a more recent chat with The Guardian he did not want to spread headlines about how he lost weight for the ‘Oppenheimer’ role.

He said when asked how he slimmed down so much: “You become competitive with yourself a little bit which is not healthy. I don’t advise it.”

His Guardian interviewer said he refuse to say how many kilograms he lost, or what food the nutritionist told him to cut out.

Murphy added: “Ach, no. I don’t want it to be, ‘Cillian lost X weight for the part.’”

But he did admit he was burning a huge amount of energy filming the project, adding: “It’s like you're on this f****** train that’s just bombing. It’s bang, bang, bang, bang.

“You sleep for a few hours, get up, bang it again. I was running on crazy energy – I went over a threshold to where I was not worrying about food or anything. I was so in it, a state of hyper… hyper something.

“But it was good because the character was like that. He never ate.”

In reality, Oppenheimer – killed by cancer in 1967 – subsisted on little more than Chesterfield cigarettes and double-strength Martinis with their rims dipped in lime.

Murphy said: “Cigarettes and pipes. He would alternate between the two. That’s what did for him in the end.

“I’ve smoked so many fake cigarettes for Peaky and this. My next character will not be a smoker.

“They can’t be good for you. Even herbal cigarettes have health warnings now.”