Tekashi69's still behind bars hoping to get out on bail, but now he has something else to worry about -- the feds announcing they've had a snitch in the rapper's crew.

We broke the story ... Tekashi pled not guilty in his federal racketeering case Monday, and a trial date's been set for September 4. Things got potentially worse for him when prosecutors revealed they are working with a confidential informant who's made recordings -- presumably of conversations with 6ix9ine.

Tekashi's attorney, Lance Lazarro, admits it's a serious development in the case, telling TMZ ... "Anytime there is a confidential informant, there is cause for concern."

In court, prosecutors boasted the recordings could send Tekashi to prison for life.

His lawyer's trying be optimistic, though, saying ... "I am confident that any recorded conversations with [Tekashi] will prove that he was never a part of the Enterprise and support the conclusion that he’s innocent."