The Daily Beast put out a story Thursday that says His Royal Highness is in poor health as he continues ongoing treatments for his cancer -- which, at this point, remains a mystery. The only thing the Palace has said is that it isn't prostate cancer ... but it's unspecified.

While KC has been seen out and about these past few months after his diagnosis -- seemingly looking healthy and robust -- the Beast reports it's dire behind the scenes.

What reporter Tom Sykes says he's been hearing from friends of the King in recent weeks is that it's "not good." In fact, it's apparently so bad ... that Buckingham Palace is said to be updating Charles' funeral plans on a regular basis these days.

Those plans are written down somewhere, per the Beast, and it has a code name ... Operation Menai Bridge. There's also this ... the Beast reports that a separate document which has dos and don'ts from Queen Elizabeth II's funeral has also been floating around lately.

If true, it's pretty stunning ... especially since Charles has been carrying on with his Royal duties and putting on a brave face in public. Based on this ... it suggests that's all for show.

One anonymous Royal insider is cited in this report as saying ... "Of course he is determined to beat it and they are throwing everything at it. Everyone is staying optimistic, but he is really very unwell. More than they are letting on."

Other sources the Beast cites in this report note that because the King has been diagnosed with cancer -- it's a no-brainer these sorts of arrangements are being looked at constantly, indicating it's a bit much ado about nothing. Still ... it's pretty eye-opening and sobering as well.