The Oscar-winner announced the news in a statement, amusingly borrowing the phrase Gwyneth Paltrow and Chris Martin used to confirm their split.
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Colin Firth has pulled out of voicing the animation film Paddington Bear
"After a period of denial, we've chosen 'conscious uncoupling'," he said.
"It's bittersweet to see this delightful creature take shape and come to the sad realisation that he simply doesn't have my voice.
"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 decision had been an amicable one. "I cannot thank him enough," Paul said of Colin's contribution.
"We love the voice and we love the bear, but as our young bear came into being we agreed that the two didn't fit. So, with somewhat heavy hearts, we decided to part ways."
The first full length trailer for Paddington was recently released online. The film is due to hit cinemas on 28 November in the UK, and Christmas Day in the US.
It stars Downton Abbey's Hugh Bonneville as Mr Brown, as well as Nicole Kidman, Peter Capaldi, Julie Walters, Jim Broadbent and Sally Hawkins.