"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," a source said.
"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 source added.
Firth's co-stars Helena Bonham Carter, who played his wife Queen Elizabeth, and Geoffrey Rush - the king's speech therapist, are also said to want to return for the sequel and director Tom Hooper is keen to take the helm once again.
'The King's Speech', which picked up four Oscars, including Best Actor for Firth, as well as a string of BAFTAs, Golden Globes and Screen Actors Guild awards, cost just 9 million pounds to make and earned over 250 million pounds at the box office.