Our sources tell us ... Jones tested positive for Turinabol -- an anabolic steroid.

Jones won the July 29 fight -- taking the UFC light heavyweight belt from Cormier with a savage head kick followed by a barrage of punishing punches to Cormier's head.

Jones will now go through the disciplinary process and his title could be stripped, which would make the fight a "no contest" and the title would go back to Cormier.

It's the second time Jones has failed a USADA drug test. He tested positive for banned substances in 2016 before UFC 200 and was pulled from the main event. Jones got socked with a 1 year ban.

We're told this time around, he could be banned for up to 4 years.

Jones had blamed his previous failed test on a male enhancement pill. He claimed he didn't realize the pill contained a banned substance and insisted he was NOT trying to gain a competitive edge.

If the test results hold up, it's unclear how he'll explain his way out of this one.

Story developing ...

7:25 PM PT -- A rep for Jon Jones released the following statement ... "We are all at a complete loss for words right now. Jon, his trainers, his nutritionists and his entire camp have worked tirelessly and meticulously the past 12 months to avoid this exact situation."

"We are having the samples tested again to determine the validity or source of contamination. Jon is crushed by this news and we are doing whatever we can as a team to support him."

7:01 PM PT -- A rep for Daniel Cormier tells TMZ Sports ... "It is very hard not to feel cheated. Again!"

6:25 PM PT -- The UFC released a statement saying they were notified today by USADA of a "potential Anti-Doping Policy violation violation stemming from an in-competition sample collected following [Jones] weigh-in on July 28, 2017" ... the day before UFC 214.

The statement goes on to say, "under the UFC Anti-Doping Policy, there is a full and fair legal process that is afforded to all athletes before any sanctions are imposed."