Michael Jackson's estate has fired back against the company that sued the singer for $300 million over the derailed Jackson family reunion concert.

You'll recall AllGood Entertainment claimed MJ promised through his rep ... the family concert would take place before any other live performance.  The AllGood suit alleges Michael breached the agreement by subsequently signing to do the London concerts before the scheduled reunion concert.

Now Jackson executors John Branca and John McClain say the so-called Jackson rep -- former manager Frank Dileo -- had no authority "to bind Mr. Jackson to an agreement."

The good news ... Janet was really good last night.