A former world No 2 heavyweight boxer once primed to fight Tyson Fury has died aged just 40, after 11 years of care following an 'attack' in which the perpetrators were never found.

Russian Denis Boytsov pulled out of a fight with Fury in December 2012 and was considered the world's second-best heavyweight behind Wladimir Klitschko. He had a 36-1 record when his career was cut short in 2015.

Boytsov suffered serious head injuries in a Berlin subway tunnel and had to be placed into a medically induced coma because of swelling on the brain.

A statement on the boxer's website suggested he was the victim of an attack linked to his boxing career. 'It is possible that this attack was linked to the boxer's sporting activity,' the statement said. 'Around two years ago, Boytsov was receiving threats constantly.'

His wife Olga Litvinova was left deeply upset and suggested Boytsov may have been the victim of an attempted 'assassination' after that series of threats. 

Now, 11 years on, Boytsov's family have revealed he died 'suddenly' this week after many years in care. His father says the cause of death was a detached blood clot.

'A blood clot broke off,' Nikolai Boytsov told Russian news agency TASS. '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.'

Brother Maxim Boytsov revealed Denis had been unable to speak in the 11 years following the Berlin incident. Boytsov underwent extensive treatment in Germany and then back in his homeland. 

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

Boytsov's funeral is set for today in the Russian region of Oryol. 

British former heavyweight boxer David Price - who once beat Fury in 2005 - revealed Boytsov's punching power was much stronger than that of Anthony Joshua, having trained with both.

'I sparred a Russian called Denis Boytsov in Germany once. He only hit the top of my head, but he was a powerful puncher,' Price told the Daily Star.

'I think he got his career cut short, he got absolutely battered and left for dead, he couldn't fight again. But I remember him being a really powerful puncher.'