His chair was kicked backwards and his head smashed on the ground, damaging his spine. The injuries left him in such intense pain that he entertained suicidal thoughts and continues to suffer short-term memory loss.

The 44-year-old star, known for his easy charm, surprised American radio listeners last week when he described the past 12 months as “the worst year I’ve ever had”, culminating in a series of operations to ease his recurrent headaches.

He said his ordeal began months ago when filming Syriana, a political thriller based on the memoirs of Robert Baer, a former CIA agent who was tortured on assignment in the Middle East. The film opens in Britain on January 13.

“The problem was that I had put on weight in about a month to do the movie. Usually I am in pretty good shape, but you should not put on 38lb when you are 44. Maybe when you are younger, but it was probably a dumb move on my part.

“There was this scene where I was taped to a chair and getting beaten up and we did quite a few takes. The chair was kicked over and I hit my head.

“I tore my dura, which is the wrap around my spine which holds in spinal fluid. But it’s not my back, it’s my brain. I basically bruised my brain. It’s bouncing around my head because it’s not supported by the spinal fluid,” he said on National Public Radio in Los Angeles.

The actor who made the leap from ER, the medical television drama, to blockbusters such as Ocean’s Eleven revealed that a family history of pill addiction ruled out conventional painkillers, so every day he had to “psych” himself into forgetting his pain.

Close associates claim doctors dismissed Clooney’s complaints until spinal fluid started leaking from his nose. Since then he has had numerous operations on the dura mater membrane.

Clooney is the latest in a long line of actors who have come to grief before the cameras. Brandon Lee, the son of Bruce Lee, died on the set of The Crow when he was shot with a gun that was supposedly loaded with blanks. Harrison Ford suffered a hernia filming Indiana Jones and the Temple of Doom.

It is not always a matter of fitness, said one Hollywood stunt director last week, but luck and skill: Brad Pitt spent months turning himself into the Greek hero Achilles for the film Troy — and then tore his left Achilles tendon.

Clooney learnt the hard way: “Before the surgery it was the most unbearable pain I’ve ever been through, literally where you’d go, ‘well, you’ll have to kill yourself at some point, you can’t live like this.’

“I don’t take painkillers because we’ve had members of our family who have become very fond of painkillers over the years,” said the actor, who has opted instead for therapy that teaches him to ignore the pain. He says he could recover fully by taking six months off work but that is not going to happen in the near future.

The injury damaged Clooney’s short-term memory, which he is trying to repair with counting exercises: “I have to work the memory muscle by counting everything, like how many times I pedal when I am on a bike.

“It’s probably the worst year personally I have ever had. My brother-in-law died of a heart attack aged 45, my grandma fell, broke her hip and died this summer. And my dog got attacked by a rattlesnake and killed,” he said.

Friends say the star felt “sick with guilt” about the dog, because he tried to beat the snake off with a stick, and worried that the pet’s last recollection may have been Clooney striking him.

Clooney, who opposes the war in Iraq, has “outed” himself as the prankster behind a fake magazine cover that had mystified the America media. He said a magazine editor had put him, alongside fellow war critics Sean Penn and Susan Sarandon, on a cover emblazoned with the word “traitors”.

So he mocked up a nearly identical “traitor” cover with the faces of other war critics, including the Pope and Pat Buchanan, the right-wing radio pundit, and sent it to 800 television stations and newspapers.

“They didn’t realise I was behind it,” said Clooney.

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