The actor appeared sans shirt in a photo posted by Jeremy O. Harris, the writer of "Slave Play" -- a show currently running in England's capital costarring Kit. And, check out the photo for your daily fitness inspiration.

Harington's wearing just pants backstage, chiseled six-pack on display for everyone in the changing room. No smile on KH's face, but it looks like he's got the smolder down.

Kit didn't post the pic himself -- and sounds like he won't be stoked about this making the internet, 'cause Harris wrote, "Kits gonna kill me for posting this. Share it so he will forgive me."

There's no real reason for Kit to worry about his thirst trap ... the internet's giving this pic rave reviews -- with many joking they're ready to hop a plane to London in hopes of hooking up with the heartthrob.

BTW ... "Slave Play" follows three modern-day interracial couples performing in "antebellum sexual performance therapy" on a Virginia Plantation -- and -- from the behind-the-scenes shots of Harington, it looks like he's embracing the challenge.

Anyhoo ... Kit slays on social media -- and, looks like he could still slay a dragon whenever duty calls!