Rumours that a film would be made were confirmed in 2008 but the future of the project has appeared unclear.
Arnett, who played George 'Gob' Bluth, has now told GQ: "Yes, it's happening."
However, he added that the script is not yet finished and explained that although all the original cast members have agreed to appear, there is no timetable for the movie.
Meanwhile, the show's executive producer Mitch Hurwitz said: "Believe it or not, we have started the script. We're taking a little abeyance while we get [new show Running Wilde] up and running. But it is our absolute next priority and we can't wait to do it."
Hurwitz added that he has a "clear path" for the movie to take and apologised for not giving fans more information.
"Believe it or not, I do not enjoy teasing our wonderful and devoted audience, which is one reason it appears that I've been so cagey about this," he said. "It's just that I don't want to promise what I can't deliver."
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