A teenager who stabbed a 17-year-old boy to death on a crowded dancefloor at a party has been found guilty of his murder.

Charlie Cosser was stabbed three times in the chest at the end-of-term party, attended by around 100 people, at a farmhouse in Warnham, near Horsham, on July 23 last year.

The teenage defendant, who cannot be named for legal reasons, faces the mandatory life sentence following a unanimous verdict in Brighton.

Three days after life support for Charlie was removed on July 25 last year, his killer pleaded guilty to murder but later changed his plea to not guilty.

Members of the Cosser family punched the air, hugged each other and wept as the guilty verdict was delivered.

The defendant showed no emotion as the verdicts were read out.

After the verdict, Charlie's dad, Martin Cosser said: "Thank God.

"We were all hoping for it but you've got to hear them say it."

The youth chose not to give evidence in the trial but denied murdering  Charlie and having a bladed article.

He originally pleaded guilty to murder during his first appearance at Lewes Crown Court on July 28 last year, but before being sentenced he successfully applied to withdraw his guilty plea, prompting the trial to take place.

Mr Cosser said: "He didn't flinch and I just wish we didn't have to go through this and he had stuck to his guns."

Lewes Crown Court, sitting in Brighton, heard Charlie was stabbed in an eruption of violence on the dance floor.

His killer threw the first punch in a mass brawl when he was asked to leave a second time after another female guest complained she was being sexually harassed by him.

Charlie's family, who have set up a knife crime charity in his name, gathered outside court after the verdict.

READ MORE: 'I saw him take his last breath': Father of teen stabbed to death tells of his agony

The court heard the defendant burned the clothes he was wearing after being driven away from the party by an adult.

He was contacted by police the next day as he ate at a carvery restaurant with his mother.

His parents, who had been in court throughout the trial, were not in court to hear the verdict.

Charlie Cosser was only 17 when his life support was switched off three days after the party.

Alan Gardner KC for the Crown said mobile phone video caught the moment Charlie, known as Cheeks, was fatally wounded.

His family sobbed as the graphic footage was shown in court.

Video dated as taken at 11.59pm on July 22 last year, shows the moment he was stabbed three times in the chest. 

The defendant or one of his friends brought the knife to the party, Mr Gardner said. 

The partygoers crowded under a marquee in the grounds of the farmhouse near Horsham as summer rain lashed down. 

Trouble started when a girl at the party complained about the defendant and said he had tried to touch her up, Mr Gardner said.

The girl said the defendant was 'creeping her out'. 

"Two separate fights erupted on the dance floor," Mr Gardner said.

"In the darkness and noise of a crowded dance floor his use of the knife and the knife blows went unnoticed," Mr Gardner said.

Charlie was able to leave the tent before collapsing on the grass.

Charlie's father clutched his head and wiped tears from his eyes as the jury heard details of his last moments of consciousness. 

"At around midnight another party guest was standing on the DJ’s stage when he noticed a boy he knew to be Charlie Cosser lying on the grass. 

"It looked like he was in discomfort. 

"He was holding his arms across his body as if he had stomach ache, and was wriggling around," Mr Gardner said. 

The court heard the defendant had blood on his hands and clothes.

Charlie Cosser, from Milford, Surrey, was described as caring and loving by his devastated parents. 

In a statement from his family, dad Martin, mum Tara, older brother Adam and younger sister Eloise paid tribute to him. 

“Our lives have been destroyed by the tragic and unnecessary loss of our son. 

"Despite courageously battling right till the very end of his young and innocent life, his injuries were too severe and he tragically passed away on the evening of Tuesday, July 25.

“He was the most caring, cheeky, loving son and brother we could have ever wished for, and we cannot imagine a life without him. 

“We would also like to thank on our and Charlie’s behalf, all of the efforts of the emergency services who treated him on the scene, as well as the whole of the critical care team at the Royal Sussex County Hospital in Brighton, who did absolutely everything they could to save our precious boy, with the upmost compassion, empathy, sympathy and care. 

“His ridiculously silly sense of humour will be deeply missed by everyone who knew and loved him.”