Jillian Michaels -- The Master Cleanse is BAD for You!!!

It's the most famous crash diet in history ... the Master Cleanse ... but according to celebrity super-trainer Jillian Michaels ... it's dangerous for your body.

Michaels was out in Malibu this weekend, when we asked about rumors that Beyonce dropped the baby weight by going on a Master Cleanse ... a 10-day program where people are only allowed to consume tea and lemonade mixed with maple syrup and cayenne pepper.

"Don't do the Master Cleanse," Michaels told us bluntly ... "It's bad for you."

Back in 2006, Beyonce told Oprah she shed a TON of poundage by using the program for 14 days ... but now, Michaels insinuates Beyonce is more educated about the dangers of the Cleanse ... and HIGHLY DOUBTS the singer would ever do it again.