The 33-year-old woman was walking home from Hemel Hempstead train station at about 8.30pm on Saturday (July 19) when she was attacked at the junction of Station Road and Heath Lane.

The offender, who had been following the woman for a short time, thrust his arm around her neck and tried to drag her into the undergrowth of an adjacent field.

He was only stopped when passing motorists who saw what was happening pulled over and started shouting.

The attacker then fled down Station Road and across the field, possibly towards Boxmoor Cricket Club or the Three Blackbirds Pub in St John's Road.

Police said the victim was 'extremely shaken but unhurt', and that a knife was later recovered from the scene of the crime.

Detective Sergeant Laurie Rush, investigating, said: "We are taking this incident very seriously and doing everything in our power to trace the offender."

He added that although such incidents were 'very rare', Herts Police is stepping up patrols near the attack scene as a precaution.

The attacker has been described by police as an 'Eastern European-looking' man in his mid-twenties about five ft 10 ins tall, of medium build, with olive skin and jet-black hair.

He was wearing blue jeans with a blue T-shirt and was carrying a distinctive rucksack with an orange pattern on it.

It is believed the offender may have been drinking in the nearby Steamcoach pub in St John's Road before or after the attack and police believe other people in the area may have witnessed the incident.

DS Rush said: "We would strongly appeal to anyone who knows anything or witnessed anything on the night to come forward."

He added: "Also, if anyone has had a similar incident in the area and not reported it, we would very much like to speak to them."

On June 21 an 18-year-old girl was sexually assaulted in Highfield, Hemel Hempstead after accepting a late-night lift home from a male stranger.

On June 20, a man exposed himself in Cowper Road, Hemel Hempstead – close to the scene of Saturday's attack – possibly to a woman and her children aged two and five.

Anyone with information about any of the incidents is asked to call Herts Police on 0845 53300222.

Police have also issued the following advice to the public following Saturday's attack:

• Plan your route and whenever possible walk with a friend
• Try to keep to well-lit, busy roads at night and avoid taking short-cuts
• Be aware of what is going on around you
• Buy a personal alarm and carry it in your hand so you can use it immediately
• Walk on the side of the road which faces oncoming traffic
• If you hear someone walking behind you, look to see who it is before they reach you
• If you think someone is following you, cross the street to check
• If you are still concerned, go to the nearest well-lit or busy place and call the police