Detectives have released disturbing CCTV footage of the moment two men attacked a man during a night out in central London, resulting in his death six months later.

Desmond O’Beirne, 51, was approached from behind as he walked through Trafalgar Square in central London at 12.25am on Saturday, June 3 last year – less than 24 hours before the London Bridge terror attack.

Police are offering a reward of £20,000 to anyone who can provide information that leads to the arrest and conviction of those responsible for the murder outside the National Portrait Gallery.

Paramedics attended the scene and the victim, who sustained a traumatic brain injury, was taken to hospital.

His injuries left him in a vegetative state and he died six months later on Wednesday, 20 December.

The victim’s sister, Vivienne Folan, said: “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.”

Folan said she thought the London Bridge and Borough Market attack, less than 24 hours later, could help witnesses remember where they were that evening.

“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,“ she added. 

Detective Chief Inspector Noel McHugh, from the Metropolitan Police’s Homicide and Major Crime Command, said: “You may have been part of the group and did not realise how seriously Desmond was hurt and that he has now died.

“That may pray 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.

“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.”

The first suspect is described as having a top knot, full beard, red t-shirt, dark trousers and light colour shoes. The suspect is seen punching the back of the victim’s head.

The second suspect was wearing a black jacket with a white emblem on the right shoulder and white writing with a picture on the rear. He was also wearing a white top under the jacket, dark trousers and dark shoes with white soles. He can be seen kicking the victim after he collapsed on the pavement.

A post-mortem examination took place at Westminster Mortuary on 3 January 2018 and gave the cause of death as a head injury. 

Anyone with information is asked to contact the incident room on 020 8785 8099. Alternatively contact Crimestoppers anonymously on 0800 555 111 or visit crimestoppers-uk.org.