Colin Firth is crowned King of the Baftas

The all-conquering movie, which had 14 nominations, also won supporting actor Baftas for Helena Bonham Carter and Geoffrey Rush, as well as prizes for music, screenplay, Best British Film and Best Film.

The King's Speech tells the story of King George VI's battle to overcome his stammer.

Collecting his award at the Royal Opera House in London, Firth said: "I like coming here. Thank you, Bafta."

He added: "I'd like to thank our masterly voice coach, Neil Swain, for indulging our dependence on him, and my very great friend Tom Ford, who has everything to do with me standing here. Also, my majestic mum and dad and Livia (his wife], who deals with everything from kings to dancing queens. Thank you very much."

Bonham Carter won Best Supporting Actress for her role as the Queen Mother, beating The Fighter's Amy Adams, Barbara Hershey for Black Swan, Lesley Manville for Another Year and Miranda Richardson for Made in Dagenham.

The actress, wearing a stately black dress by Vivienne Westwood, was presented with her award by Scottish actor James McAvoy. She said: "I'm not used to winning. King Colin, you deserve everything that's coming to you."

Rush, who won Best Supporting Actor for his role as the speech therapist Lionel Logue,. could not attend the awards as he was on stage in New York.

David Seidler, who won Best Screenplay for The King's Speech, said: "It's a small film - two men in a room - but it's spoken to the world. And for a stutterer and a stammerer to be heard, well, it's a wonderful thing."

Presenting the prize to The King's Speech for Outstanding British Film, Harry Potter actress Emma Watson said the shortlist made her "proud to be British".

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Veteran horror actor Sir Christopher Lee was honoured with the lifetime achievement award, while the Harry Potter films were given a special award for their outstanding contribution to British cinema.

Edinburgh-based Harry Potter author JK Rowling took the stage with actors Watson and Rupert Grint to receive the Bafta from comedian and writer Stephen Fry. She said: "It's very strange to look back and remember how wary I was of letting anyone put Harry on screen."

However, she added that the process had been one of the best experiences of her life.

Mike Leigh, whose film Another Year was nominated for the award, said: "I don't think there should be distinction between British films and other films. I think it has just been a great year for films." Natalie Portman won the Leading Actress award for her role as a ballet dancer in Black Swan. The film's director, Darren Aronofsky, accepted the award on her behalf, saying: "She is by far the most committed, dedicated, focused actor I have ever worked with."

The award for best British debut film went to Chris Morris for his controversial home-grown terrorist comedy, Four Lions. The satirist was not at the ceremony, but texted: "Lighters at the ready."

Firth, who won the Golden Globe last month, beat Jesse Eisenberg for The Social Network, James Franco for 127 Hours, Javier Bardem for Biutiful and Jeff Bridges for True Grit.

Firth and The King's Speech are now hot favourites for further success at the Oscars later this month. Last year, Iraq war drama The Hurt Locker won six Baftas, including best picture, and then repeated the feat at the Oscars.

David Parfitt, deputy chairman of the British Film Academy, said it had been "a very good year for the Brits".

But Britain's movie industry is under a cloud, amid an economic slowdown and government funding cuts.

The King's Speech was partly funded by the UK Film Council, a body recently abolished by the coalition government.