Robbie Coltrane has to shed seven stone before he can undergo life-changing knee surgery.
Robbie Coltrane
The 66-year-old actor has been diagnosed with osteoarthritis, which is caused by a breakdown of cartilage, but Robbie - who once weighed as much as 20st - has been told he cannot have the operation he needs in order to walk without support unless he loses weight.
Robbie - who still has another one-and-three-quarters stone to shed - said: "I'm in constant pain all day.
"I had an exploratory operation and they discovered I had no cartilage left in one of my knees.
"It's completely disintegrated. And they told me it's osteoarthritis.
"People are probably wondering why I'm hobbling about in the way I am and that is why."
Robbie is set to have an operation during which medics will use artificial components to repair his damaged knee.
He explained to the Daily Star on Sunday newspaper: "The rebuild system they use is from America and is absolutely brilliant.
"I was shown the actual thing they put in your knee and what a piece of kit it was. They glue it to the bone after shaving it flat."
But before then, Robbie knows he must shed his excess weight.
The actor said: "The equipment has a piece of rubber in the middle and if you're over a certain weight then you eventually flatten it.
"I've been trying to lose weight so I can get it fixed.
"The last time I saw the surgeon he said 'that's another 25 pounds'. I've done 84 but it's the last 25 that's the killer."
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