It was confirmed last month that a third movie in the series had been greenlit by production company Working Title Films. Speaking to PA, Firth - Mark Darcy in the franchise - confirmed that a script had now been completed.
On teaming up with Renee Zellweger (Bridget Jones) and Hugh Grant (Daniel Cleaver) - whose involvement remains unconfirmed - the King's Speech star joked: "It does become a bit of a family at the time, it'll be like one of those rather fraught family reunions at Christmas."
Rumours suggest that the film's plot will revolve around Bridget and Mark's failure to have a baby together, which leads to her having a short fling with Daniel and getting pregnant.
Firth added: "There's a script. This character has obviously inhabited part of my life. I'm always interested to see where it's taken, and it does look as if it's going somewhere interesting."
Asked if Mark would wear his infamous Christmas jumper, the Oscar winner quipped: "I wouldn't wear anything else."
Firth attended the UK premiere of his new film Tinker Tailor Soldier Spy in London last night.
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Watch the Tinker Tailor Soldier Spy trailer below: