A prison officer accused of threatening and assaulting a teenager at a youth jail has been cleared of misconduct in public office.

Alexander Williams was alleged to have thrown 15-year-old Nico Elliott against the wall of his room, pinned him down on his bed and pressed his forearm against his chest.

Elliott, now 19, told a jury: “He tried to kill me. I am choking. I am basically dying. I can’t breathe. Look at his forearms. He put them into my neck and I couldn’t breathe.”

The teenager said other staff members were shouting because Mr Williams “went too deep”.

“Normally they give you a little punch in the face,” he said. “If other staff weren’t there he would have killed me. I promise you now I would not be here.”

But a jury took just 20 minutes to clear him of the charge.

Former full-time rugby player Mr Williams, 35, wept with relief on hearing the verdict.

Judge Philip St John-Stevens told the jury of seven men and five women at the start of the trial at Maidstone Crown Court that allegations about threats and violence by staff at the G4S-run Medway Secure Training Centre in Rochester after an undercover BBC Panorama documentary had nothing to do with the case.

But prosecutor Edward Franklin said the matter was investigated when police learnt of “BBC interest in the establishment” and investigated what had happened over a number of years.

Mr Franklin said Elliott was told to go to his room on September 14 2013 after he misbehaved by arguing with another trainee.

Care assistant Louise Nicholas pressed the “first response” button to alert other staff.

One was experienced deputy operations manager Andy Williams, who sat on the bed with Elliott.

Mr Franklin said Alexander Williams, also a deputy operations manager, marched into the room and began to shout at Elliott and told him off for being rude to Miss Nicholas.

Mr Williams, he said, grabbed hold of his collar, threw him on the bed and pinned him down, pressing his forearm against his chest and shouting at him.

“This hurt Nico and made it hard to breathe,” said Mr Franklin. “He started crying.”

Another staff member, Mark Crowhurst, arrived, followed by his mother Jackie. Mr Crowhurst told Williams to let go of Elliott and pulled him off.

“Eventually, the defendant let go but remained furious,” said Mr Franklin.

Other staff members felt Williams had overreacted, rather than use approved techniques. Later, Mr Crowhurst challenged Williams about it, but he pushed his hand away and called him a “p***y and a ****”. Mrs Crowhurst reminded him Elliott was only a child and he was aggressive to her also.

A nurse later found the teenager to be angry. He punched the window in his room. He was upset and distressed.

“He didn’t make a complaint because he thought nothing would come of it,” said Mr Franklin. “There was an internal investigation and the management looked into what happened.

“The matter, putting it bluntly, was dealt with within the training centre.”

In December 2015, the police learnt of the BBC interest in the centre and officers launched an investigation covering a number of years.

“The prosecution say if you are charged with looking after young people the public would be outraged on learning you assaulted one of them.

“You are dealing with a young offenders’ institute. They are there for a reason. The prosecution case is it doesn’t give an excuse to act outside the law and assault young people.”

Mr Williams, who lives in Rugby, Warwickshire, denied the charge.

Elliott, who has convictions for violence and was brought from prison to give evidence, said he believed the man who assaulted him was called Darren. “He was huge innit - proper huge,” he said. “I think he used heavy weight lifting.

“I used to be proper small. Everyone used to call me Little Nico.”

He said of the alleged incident: “All I remember is I am saying: ‘Why am I going to my room?’ I don’t know if I am grabbed but I am slung into the wall.

“I haven’t grabbed anyone’s shirt or tie. I am being thrown into the wall and then I am being pinned down. That has happened for no reason, because this has happened to me before.

“I have been assaulted in this place more than once. I could name four occasions. It isn’t how misbehaved you are, you always get restrained.

“I know when someone’s not meant to do that. It is not two kids fighting. I have done nothing. I did box. I wasn’t boxing when I was in there.

“He shouldn’t do that. He is meant to be a carer. It is not like the adult prison. I am a child. I was upset. I was going to kill myself.”

Alexander Williams claimed in evidence he had been “set up” by colleague Mark Crowhurst.

He said the residential service manager accused him of assaulting Elliott because he had repeatedly challenged his ability to work at the “volatile” centre for 76 boys and girls aged between 12 and 17 - said to be some of the “most challenging and dangerous” in the country.

Mr Williams, whose mother is the retired head of education for G4S, said he had intervened to warn Elliott, but was grabbed by his shirt and tie.

He then took hold of the teenager’s top with both hands to prevent him from lashing out.

Mr Crowhurst claimed Mr Williams was “in a red mist” with his face red and bulging and ignored calls to stop and had to be pulled away.

But Mr Williams claimed he only landed on Elliott because Mr Crowhurst tried to pull his arm away as he gripped his top, which gave the teenager the opportunity to punch him in the face.

Mr Williams said he was on top of the teenager for less than a second before he put his arm on his chest to push himself back up.

Mr Crowhurst, he claimed, continued to pull his other arm, enabling Elliott to hit him again.

Mr Williams admitted he was angry, but it was only with his colleague.

"He said I had assaulted a trainee,” he told jurors. “I wouldn't have been in that position if he hadn't pulled me.

“His actions resulted in me being punched in the face, and then he was shouting at me: 'You have assaulted a trainee'.

"He told Nico he had been assaulted by me. Yes, I was cross. He set me up. We never got on.

"I made his life awkward by challenging his behaviour and attitude constantly through the time we were both at the centre. We didn't get on in any manner."

Mr Williams was suspended before being allowed back to continue his job.

He left the training centre, one of only four in the country, the following year and went to work at an adult prison in Rugby.

He said he now teaches control and restraint techniques to staff at the prison.

He told how he was encouraged to join the prison service by his mother after his promising rugby career was ended by him breaking his neck in 2008.

He played full-time for Rugby Lions and then Nuneaton. He was also a qualified personal trainer.

He wept as he spoke of violence by detainees at Medway on staff, who were kicked, punched and bitten. Some staff ended up in hospital as a result.