The actor, who stars as gay college professor George Falconer in A Single Man, said at the movie's UK premiere in central London that there are still "invisible boundaries" that gay actors struggle to cross.

He said: "There might be risks for a gay actor coming out. The politics of that are quite complex, it seems to me.

"If you're known as a straight guy, playing a gay role, you get rewarded for that. If you're a gay man and you want to play a straight role, you don't get cast - and if a gay man wants to play a gay role now, you don't get cast.

"I think it needs to be addressed and I feel complicit in the problem. I don't mean to be. I think we should all be allowed to play whoever - but I think there are still some invisible boundaries which are still uncrossable."

Young British actor Nicholas Hoult, who has been nominated for the Bafta Rising Star Award, said he did not feel he was taking a risk by playing the role of student Kenny, who takes an interest in Firth's character in the directorial debut of fashion designer Tom Ford.

"There was no fear (in taking the role)," said the 20-year-old.

"It was a very interesting character and when you get the chance to work with Colin and Tom, you'd be stupid to turn it down because you're scared of playing a character who's potentially gay."

Director Ford joked that he had a "crush" on Firth and Hoult and admired their acting abilities, tipping them both for awards success - with Firth expected to get an Oscar nod when the nominations are announced on Tuesday.

Firth claimed he had no plans for the announcement of the nominations, but joked he probably should have made preparations in case he missed out on a nod for Best Actor.