AceShowbiz - Jon Hamm was shocked to learn his "Tag" castmate Jeremy Renner had broken both his arms while he stepped away from the set of the action comedy to promote "Baby Driver" in London.

The former "Mad Men" star returned to the set raring to go only to learn that everyone was nursing injuries, and Renner had snapped his arms performing a stunt.

"We had several on-set injuries," Hamm tells U.S. chat show host Ellen DeGeneres. "The most notable being - I wasn't there that day, so it's impossible to blame this on me - Jeremy Renner, who broke both of his arms at the same time. Day three of production. We had 40 more days to go!

"Keep in mind this is a man who's done a hundred Avengers movies and been fine. And he does a movie about playing tag... Both arms, like firewood, just snapped like a matchstick.

"I got back (from London) and I was like, 'How's it going?' and everyone was like, 'Not great'."

But the injury to Renner didn't shut the film down - director Jeff Tomsic turned to green screen technology to complete the shoot on time: "He (Jeremy) wore green screen casts that we eventually painted out in the editing," Hamm explains, while revealing he avoided injury while making the new film - but he was reminded how unfit he is.

"I realised that running around for longer than 30 seconds would feel like I was having a heart attack," he laughs. "That was pretty much the extent of my injuries."

"I was hit in the face by a foam rubber chair... but that didn't hurt that bad," the actor recalls.

"Tag" is released next month.