Never say never ... 26-year-old AEW world champion Maxwell Jacob Friedman, aka MJF, and 27-year-old UFC star Paddy "The Baddy" Pimblett are in the middle of a full-on social media blowout.

It's unclear exactly where the beef originated ... but the men clearly don't like each other, and neither held back on Tuesday.

Pimblett, on Instagram, told MJF ... "if u wanna have a real fight let's sort it with the UFC if not pipe down! Oh tell ur boss to sign me up when yous come to Fulham's ground I be ready son"

Friedman responded, writing ... "hey bud. AEW has a show coming up in your neck of the woods. If you're man enough I'll gladly show you how real I am."

MJJ tagged UFC president Dana White and AEW boss Tony Khan.

Paddy The Baddy responded ... "I've just said that u spaceman 🤣 don't act as if ur calling for it I've just told u straight when yous come Fulham's ground (craven cottage) get ur boss Tony Khan to message my boss Dana White and see what we can sort 🤗 because I know full well you won't pass a USADA drug test to fight in the UFC 🤣🤣🤣"

The AEW champ then fired back one last time ... writing, "unlike you I'm 100% natural and I'm also 100% original. Unlike you. You're dollar store Conor McGregor talking, Will from strangers things looking ass wouldn't last two seconds in my world #Betterthanyou See you in London."

MJF won his first title Saturday night at AEW Gear ... defeating the legend Jox Moxley. Pimblett, one of the UFC's best 20 lightweights, is fighting Jared Gordon at UFC 282 on December 10. Paddy, one of MMA's most popular figures, hasn't lost since September 2018.