However, the actress is said to have told producers that she will only do the role if she can keep her current weight, according to The Sun.
The tabloid claims that the opening scene of the film will show Bridget stepping smugly onto some scales in order to show the audience that she is no longer curvy.
A source told the publication: "Renee had no reservations about saying yes once she found out she wouldn't have to get fat.
"She hated having to do that in the last two films."
Colin Firth has already said that he is keen to sign up after taking on a heavyweight leading role in the Oscar-winningThe King's Speech.