WARNING DISTRESSING IMAGERY: CCTV footage of the savage attack and the single punch which killed Desmond O'Beirne has been released  by Scotland Yard in the hope of finding a lead.

Dramatic footage of an Irishman’s last moments after he was knocked unconscious by a single punch and then kicked on the ground has been released by London police.

Desmond O’Beirne was attacked in Trafalgar Square after a night out on June 2, 2017. He was left in a vegetative state and died five days before Christmas. No one has been arrested

The CCTV shows a man delivering a blow to the head of O’Beirne who falls while another man rushes in to kick him.

Police are offering a £20,000 reward after a man was murdered with a fatal punch in Trafalgar Square. Desmond O'Beirne's family asked that CCTV of the attack was released to help find those responsible. More here https://t.co/EOLKsxKNx4 pic.twitter.com/PAHaOEiNa8

— ITV London (@itvlondon) April 9, 2018

Scotland Yard are offering a $30,000 reward for information on the killers. Detective Constable Noel McHugh said: "Someone might see this video and remember that they witnessed this incident; you will see a female who is clearly shocked by what has happened and holds her hands up to her face.

"You may have been part of the group and did not realize how seriously Desmond was hurt and that he has now died. That may prey on your mind.

"You can contact us and help us get justice. Desmond was an inoffensive man, a big man and character, who was out having a drink. The CCTV shows that he was senselessly attacked, and he never stood a chance.

Desmond O'Beirne, 51, died in Dec '17 after being assaulted in #TrafalgarSquare in June '17. Do you have information that could help officers? https://t.co/QElsJuA57F pic.twitter.com/jYchrxpyln

— Metropolitan Police (@metpoliceuk) April 9, 2018

"The events of Desmond’s attack precede the London Bridge Terrorists attacks which were in the following evening. So, whilst there is a passage of time, people will remember where they were in London, and could remember this incident.

"I’m asking anyone who took any footage or selfies around that time in to contact us; take a look through your photos and see if you have something that can help us."

Desmond’s sister, Vivienne Folan, described her late brother's attackers as "cowards".

She added: "My brother Desmond was hard working and larger than life. He was on a night out in Trafalgar Square when he was brutally and viciously attacked by two cowards who then calmly walked away and left him for dead.

"As a direct result of this attack he suffered a traumatic brain injury and was left in a vegetative state.

"Desmond’s life was ended far too soon and our family is left devastated by his loss.

"Unfortunately for Desmond, his attack happened less than 24 hours before the London Bridge/Borough Market terrorist attacks and I feel that possible witnesses may have focused their attentions on those events.

"I am appealing to members of the public who may have been in the Trafalgar Square area on the night and witnessed this assault to come forward to the police.

"I am trying to get justice for Desmond, his two attackers were very distinctive and part of a larger group. I hope that witnesses and those from that group who did not know how serious the attack was will now come forward."

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CCTV, released by the London Metropolitan Police, shows the brutal Trafalgar Square attack that killed Desmond O'Beirne.CCTV / London Police.