Chilling video footage which shows the moment a man was killed with a single punch to the head has been released by police.
Andrew Young, 40, died after being struck in the unprovoked attack by Lewis Gill, 20, in Bournemouth, Dorset, on November 6 last year.
CCTV images of the assault show Mr Young challenging a cyclist who was riding his bike on the pavement.
After one minute of discussion between the pair, Gill - an acquaintance of the cyclist - arrives at the scene.
The cyclist rides off and Gill appears to turn to follow him - but suddenly lunges towards Mr Young and launches a single vicious punch at his head.
Mr Young immediately falls on to the ground, where he lies motionless. Gill, of Sutton in south London, glances back at him then callously walks away.
Gill later pleaded guilty to manslaughter and was sentenced at Salisbury Crown Court to four-and-a-half years in prison.
Dorset Police released the CCTV images, which were shown to the sentencing judge.
Detective Inspector Marcus Hester, of Dorset Police's major crime team, said: "This was a violent attack on an innocent man which had a tragic outcome.
"This case shows that violence does not resolve anything and can have far-reaching consequences not imagined at the time.
"Today the court has issued a sentence deemed appropriate and in accordance with the evidence put to them.
"Criminal behaviour such as this will not be tolerated and we are committed to bringing offenders to justice."
The incident occurred on November 6 last year outside the Tesco Metro store on Charminster Road in Bournemouth.
Mr Young sustained a serious head injury and later died at Southampton Hospital.
Gill will serve a further six months imprisonment for other matters.