A new internet craze where people are filmed punching each other in the style of Fight Club has been blamed for the death of a 23-year-old man.
The game, known as Punch 4 Punch, involves two players taking it in terms to hit each another in the same spot until one gives up, sometimes drinking alcohol as a forfeit.
The competition, which is also called trading blows or Hit 4 Hit, is usually filmed and posted online.
The online game follows on from the Neknominate social media craze, where people filmed themselves skolling alcoholic drinks and then nominated a friend to do the same.
In the UK, Tommy Main, 23, reportedly collapsed while playing Punch 4 Punch at a party at his house in London.
The young father suffered a heart attack after being punched in the chest and died in hospital a short time later, The Telegraph reported.
Mr Main's older brother Roy, 36, has criticised people who filmed the fight on their mobile phones and didn't intervene.
"For the people that was at the party that can see this call yourself friends," he wrote on Facebook.
"I just hope that something really ... bad happens.
"You'd should have stopped it not try and film it. We live in hope.
"Hope your suffering really bad hope you can't sleep."
Detectives have said they are treating the death as "unexplained" not suspicious.
"I am especially interested in any images or footage from the night of his death," Detective Inspector Michael Norman said.
"Tommy's death is being treated as unexplained but I would urge any of his friends who were at the party on the night of his death to make contact."
A 20-year-old man has been arrested over the incident and was released on bail until September.
In the past, other online competitive challenges have included the Cinnamon Challenge, where people are filmed eating a teaspoon of cinnamon, and the Knockout Game, where people aim to make a person unconscious with a single blow.
Adolescent psychiatrist Dr Richard Graham said violent competitive games aren't new, but the internet has enhanced their popularity and increased social pressure to take part.
"This is like Fight Club online – it’s going back to the roots of masculinity and testing your strength in that way,” he told the The Telegraph.
“There’s that gladiatorial test. When your body moves from that of a child to having the full strength of adulthood, there is a need to test out and compete with others to get a sense of your potency, your strength, your courage.”
While Punch 4 Punch hasn't yet become as big a phenomenon as Neknominate on social media, videos posted online have attracted thousands of views.
Tributes have been flowing in online for Mr Main and a friend has set up a Facebook page to raise money for his family and funeral.