Firth received an Academy Award nomination on Tuesday (25Jan11) for his portrayal of stuttering British monarch King George VI in the film, while he also won a Golden Globe for his performance earlier this month (Jan11).
Screenwriter David Seidler recently admitted he originally wanted to hand the lead role to Bettany instead of Firth, and the actor has now revealed why he turned the part down.
Bettany maintains he would have enjoyed working with such a stellar cast and crew, but he needed to dedicate time to his wife Jennifer Connelly and their children, after spending five months filming The Tourist with Angelina Jolie and Johnny Depp.
He tells the Associated Press, "Number one, of course, it is a regret not to be working with such great... people, Geoffrey Rush and (director) Tom Hooper.
"I think it is a little inelegant for me to discuss it. I had been working for five months steadily and my son missed me and I missed my son. Both of my sons and my wife.
"Do I regret it? I regret not working with such amazing, amazing actors and directors and writers. But no, I have to do the right thing by my family every time."
Bettany and Connelly are currently expecting their third child, while the actress is also mother to a daughter from a previous relationship.