Kim Jong-un might not be alive after all and instead has a convincing body double -- so says one former member of parliament ... who appears to be pushing actual fake news.

Louise Mensch -- who used to serve in the UK government as a conservative a while back -- sent the body double theories into hyper-drive on Friday after she posted a would-be, side-by-side comparison that she claimed proved the guy who took that factory tour in Pyongyang last week wasn't actually KJU -- but another person entirely posing as him.

She pointed out the different brow, and other facial features ... but much of her claim was on the teeth themselves. In her photo, they certainly do look different. The problem ... her photo of Kim at the fertilizer plant isn't authentic and is almost certainly a doctored image.

We know this because we downloaded the high-res image itself and zoomed in with our own gear -- and it does NOT show what this woman's pic depicts ... namely, mangled and mashed up teeth that are vastly different from Kim's famous Bugs Bunny-like chompers.

As you can tell in an actually zoomed-in photo, those two long incisors are the same here ... it definitely looks like Kim -- and not whoever that other guy is up there in Louise's comparison.

That said, we still don't know when these photos were actually taken -- and it's hard to take government-run media there at their word that he's a perfect picture of health ... especially after international reports pegged him as either dead, brain dead or on his death bed.

We'll keep an eye out for Kim ... real or otherwise.