The boxing champ was a guest on the latest instalment of the ITV morning show and host Shephard was challenged to stand 20 seconds of body punches.
He was game when it came to giving it a try – bit admitted he felt a bit worried.
"I think he's knocked out 33 people – I feel a bit queasy," he said.
As they got started, Shephard's co-presenter Kate Garraway reassured him by saying: "Just to say, Dr Hilary is on standby.”
The TV star and Eubank Jr then faced off, with Shephard quipping: "Have you ever sparred with somebody in a shirt and tie before?”
“This is new!" said the boxer. "I thought I’d seen and done it all before. Turns out I haven't!”
He then proceeded to pummel the presenter for 20 seconds, while Garraway and fellow presenter Ranvir Singh squealed.
Shepard, who was wearing some padding around his waist, groaned and grimaced as Eubank Jr served up a volley of punches.
"Longest 20 seconds of my life!" he exclaimed.
After it had finished, he told the others: "That went on for about three days!”
The boxer was impressed with how Shephard had done and invited him to his gym for "a proper spar".
Viewers loved the hilarious segment.
"Thanks to @benshephard and @ChrisEubankJr for providing a funny bit of TV on #GMB today," one tweeted.
"It’s good to start the day off with laughter."
"You know @ChrisEubankJr went easy on @benshephard," joked another.
Good Morning Britain airs weekdays from 6am on ITV1 and ITVX.
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