The actor is said to have signed up to reprise his Oscar-winning role of King George VI for the project, The Sun claims.
The film will allegedly be set during the Blitz in World War II, with director Tom Hooper and stars Helena Bonham Carter and Geoffrey Rush expected to return.
However, the story will not follow George VI's continued struggle with his stammer, but rather the differences between his and speech therapist Lionel Logue's families during the war.
An insider said: "The sequel is going to be about the experience of families during the Blitz. It will focus on how the privileged Royal Family was hit by the crisis, compared to the more ordinary family of George VI's speech therapist.
"The focus will be on George, but it will also show how the whole royal household was affected. There's still amazing interest in the Blitz. The movie is still in the very early stages, but everyone's keen to get going soon."
The King's Speech won four Oscars in 2011 for 'Best Picture', 'Best Director' for Hooper, 'Best Actor' for Firth and 'Best Original Screenplay' for David Seidler.
Watch the trailer for The King's Speech below: