According to new legal docs, obtained by TMZ, the judge said among the several reasons why the disgraced singer will NOT get out of jail is because he's still a flight risk. The judge said "[Kelly] is currently in custody because of the risks that he will flee or attempt to obstruct, threaten or intimidate prospective witnesses. The defendant has not explained how those risks have changed."

As for Kelly's fears he'll catch COVID-19 if he remains behind bars ... the judge said there are no confirmed cases of COVID-19 at MCC in Chicago where Kelly's penned up. The judge also points to measures taken by the Bureau of Prisons in combating the spread of the virus in its facilities ... like suspending all social visits and suspending inmate facility transfers.

What's more ... the judge says she ain't buying Kelly's part of the at-risk group 'cause he "is fifty-three years old, twelve years younger than the cohort of 'older adults' defined by the CDC as at high risk."

Kelly had argued he's had surgery while in jail, but the judge said, "he does not explain how his surgical history places him at a higher risk of severe illness. Moreover, officials in Chicago have advised the government that doctors have completed all treatment for the defendant’s recent operation."

As for Kelly's argument, he can't meet with lawyers to strategize for his July trial ... the judge said Kelly can still communicate via phone and email. And get this ... the judge said it's unlikely his trial will proceed as scheduled in early July ... adding "as conditions return to normal, [Kelly] and his lawyers will have additional time to prepare for trial."

We broke the story ... Kelly claimed the conditions he's under while in prison are "tantamount to making them drink poison." He is currently behind bars in Illinois as he awaits his trial, this after getting slapped with a slew of charges by federal prosecutors in NY.