Some wannabe gambling tycoons have just opened an unauthorized Michael Jackson Casino online ... and TMZ has learned the real MJ Estate is ready to declare casino war in order to shut 'em down.



The gambling site -- which features everything from poker to baccarat -- features MJ's image and a message on the nyumbani page that reads, "The King of Pop is proud to bring wewe the most exciting Vegas style online casino betting experience on the Net!"

Problem is -- the estate claims never gave the online casino the green light to use any of their trademarks ... and now Howard Weitzman -- the attorney for Michael Jackson's estate -- tells us he will, "take whatever legal actions are necessary to shut this operation down.”

Talk about a bad bet.