Police are investigating a "nasty attack" where a man was briefly knocked unconscious after being punched in the back of the head.

Hertfordshire Constabulary says it is making enquiries into an assault outside the Watersmeet Theatre in Rickmansworth High Street on Friday, July 10.

The force said the victim was walking with several friends when they were approached from behind by a group of approximately four men at 11.10pm.

One of the men punched the victim - a man in his 20s - in the back of the head, causing him to fall to the floor and momentarily lose consciousness, according to police.

He was taken to hospital for treatment and has since been discharged.

Police say that one of the men in the group that approached the victim and his friends was wearing a white top with a distinctive black design on it.

Detectives investigating the assault are now appealing for information and witnesses.

Detective Constable Paul Burton, from the Three Rivers Local Crime Unit, said: “This was a nasty attack and the victim suffered quite a serious head injury from which he is now thankfully recovering.

“We believe the High Street would have been fairly busy with members of the public making their way home from a night out. Did you see this assault? Perhaps you were driving past at the time?

“If you have any information – or any dash cam footage – you believe could be helpful, then please get in touch as soon as possible.”

You can email DC Burton at paul.burton@herts.pnn.police.uk or call the non-emergency number 101 quoting reference 41/54179/20.