By TMZ Sports

Another huge blow to 50 Cent's boxing promotion company -- because the organization's biggest boxing star, Yuriorkis Gamboa, officially wants out ... TMZ Sports has learned.

The former WBA and IBF featherweight champ says he's had issues with SMS Promotions for months ... claiming the company was contractually obligated to deliver 3 fights per year ever since he came on board in 2012 ... but SMS has failed to live up to those terms.

In fact, Gamboa only fought once in 2013, twice in 2014 ... and has no fights in 2015.

It's all outlined in court docs filed by Gamboa in Connecticut and obtained by TMZ Sports ...in which Gamboa says his team contacted SMS to negotiate the termination of his contract earlier this year, but he got no response.

Now, he wants a judge to force 50's company to answer the phone and let Gamboa know if they're gonna let him go ... or try to deliver the fights.

It's a far cry from what Gamboa said back in May -- when he pledged to stick by 50 Cent after the company filed for bankruptcy.

There's more ... another SMS fighter, Ryan Martin, has also filed docs claiming SMS failed him too ... so he wants out.

We reached out to 50 Cent for comment -- so far, no word back.