And while spouting support for IS and hatred for Western culture, he was known as Beyonce Boy in prison because of his love for the US pop superstar.

Last night campaigners said it showed the “empty and pathetic” ideology at the heart of Islamic terrorism.

Khan, 28, who also whined he had been bullied at school, ballooned to over 18st by gorging on fast food while living alone following his early release from prison.

The knife-wielding fanatic murdered Saskia Jones, 23, and Jack Merritt, 25, on Friday before being tackled by brave bystanders and then shot dead by police.

But even though he had been planning slaughter for months, he shelled out £1,000 for slimming treatments at a health clinic close to his terror lair in Stafford.

Sources said he had a series of £350 fat-freezing procedures which can reduce fat in the treated area by up to 35 per cent.

It was also revealed Khan, jailed in 2012 for plotting to blow up the Stock Exchange and kill Boris Johnson, would secretly watch Beyonce videos on his cell TV.

Former North West England chief prosecutor Nazir Afzal said: “This 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.

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

Dr Paul Stott, of foreign policy think tank the Henry Jackson Society, commented: “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.

“Like many attention-seeking terrorists, his vanity came before the warped beliefs he claimed to represent.”

FAT-FREEZING TREATMENT

The Sun can reveal jobless Khan had the slimming treatment over several weeks while chatting to staff about his hopes to improve his life after whining he had been bullied.

The procedure freezes fat cells which then pass out of the body naturally.

Clients generally report seeing changes to their waistline in just a couple of weeks and can keep off the unwanted inches if they maintain a healthy lifestyle.

A senior clinician at the centre said: “There were a number of treatments. We’d even been in discussion with his GP over an issue he’d been bullied about at school.

“He lost a lot of weight after the treatments. He appeared happy and talked about looking forward to finding a job.”

Staff were stunned when they found out he had knifed two victims and injured several more. They recognised him when he was named and pictured in the media.

The clinician added: “We had several consultations with him over a number of weeks but there was no indication whatsoever that he was going to do what he did.

“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?

“I’ve worked with mental health patients and he showed no signs that he was suffering with this.

“He also gave no clues that he had been in jail or any previous crimes.”

Other former inmates told how the terrorist loved watching the 4Music channel when he thought no one else was around.

A source who was in Belmarsh with him said: “Most radical Muslims in prison wouldn’t watch their TV and would instead study Islam, so Usman hid his TV under his bed and would watch 4Music late at night.

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

Other sources say he was plotting his attack long before he was released early from Whitemoor Prison, Cambs.

'HOLIDAY CAMP FOR JIHADIS'

One told The Sun: “He kept saying, ‘They’re gonna be p****d off when I get out.’ He was very angry at the police and the government.

“He said he wanted to do ‘some kind of attack’ when he got out -he never went into much detail but he was really serious when he mentioned it.

“HMP Whitemoor is like a training camp for extremists in the UK.

“Radical Muslims get excited to be sent there so they can talk and plot with like-minded people. It’s like a holiday camp for jihadis. It’s ridiculous.”

Khan grew up in Stoke-on-Trent, where he was seen waving a huge al-Qaeda flag at an extremists’ rally in 2010.

He was later caged until December last year after being convicted of terror offences.

Khan, said to have a wife and young family who lived separately to him, was told to serve at least eight years of an indeterminate sentence.

That meant he could not be freed until he was no longer deemed a danger.