THE evil plot to murder and castrate pop star JUSTIN BIEBER was motivated by a deranged killer’s twisted gay fantasies, The ENQUIRER has learned exclusively.

Convicted murderer Dana Martin, who’s locked away in a New Mexico prison, hatched a plan to kill the 18-year-old “Baby” singer because he didn’t want anyone else to have him, police sources say. “Martin is completely obsessed with everything Jus­tin Bieber,” a source told The ENQUIRER. “He actually believes they had some form of a budding romance that was sparked by the dozens of letters he’s mailed to one of Justin’s fan clubs.

“Martin’s obsession is nothing short of creepy – he has a tattoo of Justin etched into his leg and photos of him strewn around his jail cell.”

Martin, 45, is currently serving two consecu­tive life sentences in a Las Cruces, N.M., prison for the 2000 rape and murder of a Vermont teen­ager. But despite his incarceration, sources say he was convinced that he and Bieber would one day be involved in a romantic relationship.

“Martin used to tell his fellow inmates that if he couldn’t have Justin then he was going to make sure nobody could,” the source revealed. “Authorities now believe seeing photographs of Justin and girl­friend Selena Gomez together might have triggered him to hatch the murder plot that was just a few days away from coming to fruition.”

Martin allegedly enlisted former fellow inmate Mark Staake to carry out the murder after Staake was released from prison. Cops say Staake, 41, then recruited his 23-year-old nephew, Tanner Ruane, as an accomplice in the twisted plan to strangle and castrate both Bieber and his bodyguard at one of the singer’s two Madison Square Garden shows in late November.

According to an official affidavit The ENQUIRER obtained from the New Mexico State Police, cops were tipped off to a Nov. 19 phone call between Martin and Ruane.

“Reference was made to the method of killing, which was the use of a neck tie, similar to the way in which Mr. Martin killed his victim(s),” the affidavit states.

“When Mr. Martin asked what Mr. Staake had bought to castrate the victims, Mr. Ruane stated that he bought a pair of hand(held) hedge clippers… Mr. Martin confirmed that Mr. Ruane was going to get $2,500 per testicle.”

But Martin apparently had second thoughts about killing his dream lover and blew the whistle on his accomplices, and Staake was arrested on outstand­ing warrants before he was able to carry out the sinister plot.

“Dana Martin is obviously a very dangerous and delusional man,” added the source. “Justin’s handlers are afraid of what else he’s capable of plotting behind those prison walls.”