The BIEB’s drug shame exposed! PLUS – His forbidden relationship  with underage fan!

IF photos of Justin Bieber smoking pot weren’t enough to rattle his de­voted fans to the core, The ENQUIRER’s latest discovery will surely shatter whatever’s left of the “Boyfriend” singer’s tattered public image!

In a bombshell exclusive, we’ve learned that the 18-year-old superstar cheated on longtime former girlfriend Selena Gomez with a Mem­phis teenager, carrying on a sizzling relationship that included late-night hookups in hotel rooms and regular use of mari­juana. Pals say the girl also boasted that she used the “designer drug” ecstasy.

“The last time they saw each other was in November, after Justin’s Memphis con­cert, and he invited her and one of her classmates back to his room to party,” a source close to the girl told The ENQUIRER.

“Later that night, when the small party finally broke up, the girl stayed behind to be alone with Justin.

“I think she’s in love with him and is holding out hope that they may end up together.”

The ENQUIRER has chosen not to identify the girl by name, but in an exclusive interview, her father confirmed that his daughter has had a three-year “friendship” with the singer since she was 14. He added that the two smoke pot on a regular basis when Justin is in town.

Although Bieber’s camp denies he has a girlfriend in Memphis, the teen has bragged to friends that she’s been dating him, according to the source. She also claimed that they’d smoked weed and she said she often took ecstasy “to make her­self feel sexy.” She recently posted a photo of her and Justin on the social networking site Instagram, but the image was quickly deleted after she received a flurry of negative feed­back from family and friends.

Although The ENQUIRER has seen the photo, we’ve decided not to publish it due to privacy concerns.

Justin and former Disney star Selena, 20, began their on-again, off-again romance two years ago, splitting for good in November over scheduling conflicts and “ma­jor trust issues” on Selena’s part.

“But Justin and his Memphis girlfriend date back to before he and Selena were together,” the source maintained.

“His teen girlfriend says when­ever she’d bring up Selena’s name, Justin would just shrug his shoul­ders and dismiss it, claiming he never loved Selena and that they weren’t in an exclusive relation­ship.”

According to the source, Justin and the teen continue to talk on the phone, text and Skype on a regular basis, but “it’s pretty obvi­ous he’s just leading her on.”

The latest blow to the Bieber brand comes just weeks after photos surfaced of Justin alleged­ly smoking a marijuana joint in a Newport Beach, Calif., hotel room with rapper Lil Twist on Jan. 2.

Although Bieber’s mom, Pat­tie Mallette, vehemently denies it publicly, sources say she wants to get him into rehab.

Another insider says that Pattie recently demanded a secret meet­ing with Justin’s manager, Scooter Braun, and his team of handlers and insisted they take action.

“Pattie’s worst fear is that Justin will end up just like his father,” said the insider. “He was a booz­ing, brawling drug user who walked away from his family when Justin was a baby.

“So I’m sure she will locate the best addiction specialists in the field and demand that a plan be put into place immediately. Pat­tie’s terrified if she doesn’t step in now, Justin will become another Hollywood tragedy.”