The King's Speech actor was due to voice the famous Peruvian bear in the film Paddington, also featuring Nicole Kidman and Peter Capaldi, but he and the director announced that they both agreed that his voice did not fit.
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The 53-year-old British actor used a term introduced by actress Gwyneth Paltrow and husband Chris Martin when they recently announced their separation, to reveal that he and the movie had parted ways.
"It's been bittersweet to see this delightful creature take shape and come to the sad realisation that he simply doesn't have my voice," he said in a statement to Empire.
"After a period of denial, we've chosen conscious uncoupling.
"I've had the joy of seeing most of the film and it's going to be quite wonderful. I still feel rather protective of this bear and I'm pestering them all with suggestions for finding a voice worthy of him."
The film's director Paul King said the split was amicable and he had not yet selected a replacement for the Love Actually actor.
"Colin Firth has been a joy to collaborate with and I cannot thank him enough for his contribution to Paddington. We love the voice and we love the bear - but as our young bear came into being, we agreed that the two didn't seem to fit," he said.
"So, with somewhat heavy hearts, we decided to part ways. We are excited about the next chapter for Paddington, ensuring his big-screen debut is magnificent."
A trailer for the film, which also features Sally Hawkins and Hugh Bonneville and is due to open in the US for Christmas, has already been released.
The duffel coat-wearing, marmalade-loving character, who starts a new life in London, was created by Michael Bond for a series of books,
The film, due out in the UK in just five months on November 28, sees the bear despatched from his native jungle by his aunt Lucy, smuggled aboard a boat to England and named after the London station where he is found.
His misadventures with his adopted Brown family see him trying to evade a sinister taxidermist (Kidman) who has her eye on him.