IG has officially slapped the Mother's Day picture seen 'round-the-world -- which has since been revealed to be heavily edited -- with a flag that explicitly lets people know the picture ain't authentic ... and that users would be wise to be cautious with it.

The disclaimer on her post now reads, "Altered photo/video. The same altered photo was reviewed by independent fact-checkers in another post."

A further description of their decision to label her picture with this explains that the fact-checker found the photo had been edited in a way that could mislead people, but not because it was shown out of context. Translation ... this thing was cooked up in the lab!

The third-party fact-checker that IG (Meta) is relying on for this conclusion is a company called EFE Verifica -- which is a Spanish org that specializes in this sorta thing.

BTW, Instagram is not the only company that's distancing itself from Kate's pic -- French news agency AFP (a well-respected player) came out publicly this week to say that they do not consider Kensington Palace a trusted source.

AFP's global news director said they were going to heavily vet anything that Kensington sends their way going forward ... and a notice has been sent to their teams globally.

It's a big hit to the reputation of Kensington Palace, no doubt -- and the fact that they still have this doctored image up is astounding ... especially with all the backlash, conspiracy theories and speculation that has followed on Kate's whereabouts and condition.

Of course, this head-in-the-sand approach -- remember, the Palace has yet to address more questions about this, despite everyone having reached out for further comment -- seems to be spearheaded by Prince William himself, who's continuing on like nothing's happened.

In fact, Bill was on hand Thursday for more Royal duties with the OnSide Youth Zone -- who help disadvantaged kids -- and here, he was seen making cookies with youngsters ... and he actually alluded to Kate, saying his wife was the "arty" one as he botched icing decorations.

He's been doing a bunch of work-related stuff all week, and has yet to address head-on what's up with Kate ... frankly, it's unclear if anyone's even asked him.

If his attitude here -- namely, to ignore and power on -- is any indication ... it seems the rest of the Kensington Palace team is following his lead, but Kate-Gate ain't going away.