The Mirror dropped this bombshell Tuesday, claiming they've been told that at least one staffer at the London Clinic had attempted to access Kate's personal file -- presumably to see what was actually wrong with her -- but the outlet says they were intercepted in the act.

There's not much more detail in terms of how exactly this unfolded, or what happened to the worker who allegedly did this ... not to mention who saw them and put a stop to it.

In any case, the Mirror says an internal investigation has been launched at the London Clinic -- and they claim top brass at the hospital contacted Kensington Palace once they became alerted to the attempted security breach, and promised they'd get to the bottom of it.

The Mirror also reports that other workers there are shocked and shaken up by this -- as the London Clinic is a well-respected and usually honorable medical establishment that treats high-profile people ... including members of the Royal family at large.

The London Clinic told The Mirror, "We firmly believe that all our patients, no matter their status, deserve total privacy and confidentiality regarding their medical information."

No word on whether this has been reported to cops either -- but from the sounds of it, it hasn't. The Metropolitan Police in London tells us ... "We can’t provide anything on this, as we need to know if the London Clinic have reported it to us and, if so, when."

Remember, this is the same hospital Prince William was seen leaving in January -- shortly after Kate was hospitalized to undergo an abdominal surgery ... for a mysterious ailment.

There was a lot of questions about her health/condition then -- but not nearly as much as there is now considering she's been MIA for months ... only just surfacing again Saturday.

That's why it's shocking to hear this, assuming it's true. On its face, it seems at least one staffer was gravely curious to see what was up with Kate ... risking their job and reputation to steal a glance at her chart at the time, when most everyone was content to leave her be.

Of course, a lot has happened since Kate left this hospital -- including lots of conspiracy theories on her health and whereabouts, which have reached peak levels of absurdity.

Kensington Palace, of course, has been fairly mum over just about all of it. We've reached out to the London Clinic and the Palace on this new story ... so far, no word back yet.