Nick Carter's Bandmate Brian Littrell Scoffs at His Sexual Assault Accuser's Claims

Backstreet Boys singer Brian Littrell is backing up his bandmate Nick Carter in his ongoing legal war with his sexual assault accusers ... TMZ has learned.

Nick filed a ton of evidence in court this week as part of his court battle with accusers Melissa Schuman and Shannon Ruth ... and the docs include a declaration by Brian.

Shannon, who has autism and cerebral palsy, claims Nick assaulted her on a tour bus in 2001 after a Washington concert. She says Nick gave her a drink that she believes was filled with alcohol … and then forced her to perform oral sex. She claims she cried as the alleged assault happened.

Nick adamantly denied the claims … and believes Melissa and Shannon conspired together to push false claims in an attempt to shake him down.

In his declaration, Brian wrote, "In the 2001 timeframe, the Backstreet Boys were world-renowned and at the height of their fame."

He said they were on their "Black & Blue" tour in 2001, adding, "Due to our group's immense popularity at the time, security was extremely tight throughout the entire 'Black & Blue' Tour."

He said for safety reasons, "my band members and I were not allowed to go anywhere, including the bathroom, unless we were accompanied by security."

He claims the "only time my band members and I were left alone was in the privacy of our own hotel rooms" and said they were isolated "from the rest of the world" during the tour.

He said the bandmates also traveled together as "one unit." Brian said his understanding of "the security protocol" for the tour would not have allowed for Nick to be left alone with a random fan on a bus.