The actor has decided to pursue new projects after playing the Sheriff of Nottingham on the BBC One drama since its launch in 2006.
Speaking to the South Wales Evening Post, Allen explained: "I doubt I'll go back for a fourth series if they do one - it's boring to work on now. I've done three series, and I'd like to move on to something else."
He added: "The work I've done on it has been brilliant, though. I've enjoyed all three series a huge amount, they've been excellent.
"But there's lots of different aspects to performance, and I want to try different things."
Lead actor Jonas Armstrong has already announced that the current series will be his last.
Though a fourth run has not been ruled out, the BBC has not yet commissioned new episodes of the programme.