A former world heavyweight contender boxer has died aged 40, after 11 years of care following an 'attack' in which perpetrators were never brought to justice.

At the peak of his powers, Denis Boytsov was ranked as high as No.2 in the world and considered by many critics to be the best heavyweight behind former world heavyweight champion Wladimir Klitschko.

Boytsov was scheduled to face Tyson Fury in December 2012 but was forced to withdraw from the contest.

He made his pro debut in 2004 and won 33 professional fights in a row before facing Alex Leapai. His career was cut short in 2015, and he finished with a 36-1 record.

Statements on Denis Boytsov's Death

In 2015, Boytsov was found unconscious in a Berlin subway tunnel. He suffered serious head injuries and had to be placed in a medically induced coma for seven years because of swelling in the brain, prompting him to retire from boxing.

He lived in Germany for more than 10 years before the accident and fought most of his fights there. He underwent extensive treatment in Germany before returning to Russia, where he received more treatment.

A statement was released on the Russian's website in the wake of his death and suggested he was the victim of an 'attack' linked to his boxing career.

It read: "It is possible that this attack was linked to the boxer's sporting activity. Around two years ago, Boytsov was receiving threats constantly."

The World Boxing Association (WBA) added: "The WBA deeply mourns the passing of Denis Boytsov at the age of 40. A former WBA Intercontinental champion and ranked contender. The WBA extends its heartfelt condolences to his family, friends, and loved ones during this difficult time. May he rest in peace."

His wife, Olga Litvinova, also suggested Boytsov may have been the victim of an attempted 'assassination'.

Denis Boytsov's Father Reveals Cause of Death

Boytsov's family have revealed he died 'suddenly' after many years in care, with his father saying the cause of death was a detached blood clot.

Speaking to Russian news agency TASS, Nikolai Boytsov said: "A blood clot broke off. There was nothing they could do; it was instant death. Three minutes and that was it.

"This happened at home. His wife fed him and started turning him to the side. He suddenly broke down, started shaking, turned blue. And his whole life was over."

Maxim Boytsov revealed his brother had been unable to speak in the 11 years following the Berlin incident.

"No, we didn't communicate," he said. "He just nodded yes and no. But he understood everything. I courted him for ten years. But it didn't work out."