The 53-year-old weather presenter is standing in for Laura Tobin on the ITV breakfast show and host Richard Madeley revealed people had been messaging in all morning to say they recognised him from the BBC soap.
Coleman played barman Lenny Wallace, who had a fling with Bianca Jackson, played by Patsy Palmer, back in 1997.
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After producers showed a picture of the actor and singer in EastEnders with Palmer he said: "That was my first ever screen kiss. You'd actually think that kissing someone would be quite romantic but with 50 people in the room it isn't and the director screaming in your ear 'You're doing it wrong, you're doing it wrong'.
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"Changed my kissing style after that, so thanks for that. I still don't think I'm doing it right, actually."
Madeley, 66, suddenly recalled: "Didn't I interview you, on This Morning?"
Coleman said: "The first interview I ever did was on This Morning with you, and I think I've still got the VHS tape, somewhere. And you're still as good looking as you were back then Rich."
Madeley replied: "Get out of here! You haven't changed a bit mate. Amazing. Alright then, do you want to act the weather?"
One viewer tweeted: "Just turned on #gmb and Lenny from Eastenders is on it as the weatherman. Very confusing."
Another said: "That's Lenny init? From Eastenders? Des Coleman #gmb @gmb"
Coleman starred in EastEnders between 1996 and 1999. He has appeared in The Bill, Casualty and Doctors as well as starring on the West End stage.
He trained in meteorology in 2007 and began reporting the weather on East Midlands Today.