Crowning moment for Colin as King's Speech rules Oscars
Firth told the audience at the 83rd Academy Awards at the Kodak Theatre in Los Angeles he wanted to leave the stage before he embarrassed himself by dancing with joy.
He said: "I have a feeling my career has just peaked. My deepest thanks to the academy.
"I'm afraid I have to warn you that I'm experiencing stirrings somewhere in the upper abdominals which are threatening to form themselves into dance moves which joyous as they may be for me, it would be extremely problematic if they make it to my legs before I get off stage."
The story of King George VI's battle with his stutter also won Best Picture, Best Director for Tom Hooper and Best Original Screenplay for David Seidler.
Firth thanked his British fans, paying tribute to "all the people who have been rooting for me back home" and thanked his wife, whom he said had put up with his "fleeting delusions of royalty".
London-born Seidler dedicated his win to "all the stutterers throughout the world".
He said: "My father always said to me I would be a late bloomer. I believe I'm the oldest person to win this award."
The team behind the film was presented with the Best Picture award by Steven Spielberg and producer Iain Canning thanked the "best of British crew" and the film's "acting royalty".
Co-host James Franco said the film, which had been nominated for 12 awards, had entered "Oscar history" before the ceremony ended with a performance of Over the Rainbow by a school choir.
Kirk Douglas presented The Fighter's Melissa Leo with the Best Supporting Actress Oscar and Welsh-born Christian Bale picked up Best Supporting Actor for the same film.
Bale, who first hit the headlines as the child star of Empire of the Sun, said: "Bloody hell. What a room full of talented, inspirational people and what am I doing in the midst of you?"
Last year's Best Actor winner Jeff Bridges presented the award for Best Actress to Natalie Portman for her role in the dark thriller Black Swan.
The tearful star paid tribute to the other actresses that were up for the award, saying she wished "the prize tonight was to get to work with my fellow nominees".
She also spoke about the father of her unborn child, Benjamin Millepied, whom she said had given her the "most important role of my life".
The ceremony had started badly for The King's Speech when it missed out on two early Oscar opportunities.
It lost out to Alice in Wonderland for Art Direction and sci-fi thriller Inception picked up the Cinematography award.
Aaron Sorkin picked up the Best Adapted Screenplay award for his work on the Facebook film The Social Network.
Bristol-based graffiti artist Banksy lost out in the race for the Best Documentary award, whilst London-born Christopher Nolan saw his film, Inception, miss out on the big prizes - but it still picked up four Oscars.