James Corden might be Mr. Nice Guy on camera -- but, according to one restaurant owner, the dude's been incredibly nasty on more than one occasion while dining.

The 'Late, Late Show' host was in the crosshairs of restaurateur Keith McNally -- who owns the highly acclaimed Balthazar in NYC, and who went on the attack Monday on IG ... calling JC a "cretin of a man" for what he claims was abhorrent behavior at his establishment.

McNally alleges that James was incredibly abusive to his staff on at least two occasions while eating there -- so much so, in fact, he felt compelled to out him publicly ... something that's almost never done by high-end restaurant owners.

The guy details two alleged incidents in which he says James lost his cool to the point of absurdity. In one alleged instance, McNally says James demanded two free rounds of drinks for him and his friends after he presented a hair that was found on his food.

McNally claims James said, "Get us another round of drinks this second. And also take care of all of our drinks so far," adding, "This way I (won't) write any nasty reviews in Yelp or anything like that."

In another alleged episode, McNally claims James went even crazier on employees. He alleges James flipped out when an egg yolk omelette his wife had ordered was found to have a little egg white in it. McNally says James told the staff, who remade it ... but mistakenly sent it back out with fries instead of a salad like his wife had initially asked.

McNally claims James went ballistic, allegedly tearing into the waiter and saying they couldn't do their jobs -- and even allegedly suggesting he go back into the kitchen to cook it himself.

Corden has now been banned from Balthazar.

James -- a big-time foodie -- was apparently able to be appeased in the end, McNally claims, but he goes on to allege James has acted similarly at another restaurant of his as well in the city.

We've reached out to James' camp for comment ... so far, no word back.