London Bridge killer Usman Khan paid $1,300 for slimming treatments just days before his deadly terror attack — and was dubbed “Beyoncé Boy” because of his forbidden love for the diva, it was claimed Wednesday.

The terrorist killed by cops on London Bridge had a series of fat-freezing procedures after ballooning to 250 pounds — with the last treatment shortly before he killed two in a stabbing spree last Friday that he had been planning for months, according to the Sun.

He complained to his doctor about being bullied — and was dubbed “Beyoncé Boy” in prison by inmates who found him hiding a TV to watch pop music shows while pretending to study Islam, the paper said.

Khan “appeared happy” after losing weight through the treatments, a senior clinician at the health clinic in Stafford told the paper.

“There was no indication whatsoever that he was going to do what he did,” the fat-loss clinician told the Sun, saying he showed no signs of being mentally ill.

“None of what he was doing makes any sense to me — why would he work so hard on his wellbeing and then carry out an atrocity like that?”

Khan hinted about plans for an attack before he was released early from Whitemoor Prison, where he was serving time for an earlier terror plot to blow up London landmarks.

He was also teased over his unexpected love for pop music while in the prison, which a source claimed is “like a training camp for extremists in the UK.”

“Most radical Muslims in prison wouldn’t watch their TV and would instead study Islam, so Usman hid his TV under his bed,” a source said, with him favoring a pop music show.

“Some of the older guys found it quite funny and would tease him. They started calling him ‘Beyoncé Boy’ after they caught him watching her Bonnie and Clyde video one evening.”

Khan killed Jack Merritt, 25, and Saskia Jones, 23, and injured three others before he was tackled by onlookers and finally shot dead by cops. He was wearing a hoax suicide bomb vest because he wanted to be shot, authorities said.

Former North West England chief prosecutor Nazir Afzal said the report “shows what a pathetic coward” Khan was.

“He was more concerned about his waistline than his victims. It shows what an empty ideology Islamist terrorism really is,” Afzal told the Sun.

“Terrorists profess to hate this country but are secretly enjoying cosmetic surgery on their fat stomachs and listening to Beyoncé in their prison cell.”

Dr. Paul Stott, of foreign policy think tank the Henry Jackson Society, said it showed that “his vanity came before the warped beliefs.”

“Far from being a battle-hardened holy warrior, Khan seems to have spent a fortune on losing his prison paunch so he looked svelte for the TV cameras as he butchered innocent Brits.”