The incident happened at about 8.30pm on Saturday, July 19, when a 33-year-old woman walking home from Hemel Hempstead train station was grabbed round the neck by a man who tried to drag her into a field.

The attacker, who struck at the junction of Station Road and Heath Lane, was only stopped when passing motorists who saw what was happening pulled over and started shouting.

He ran off across part of the field used by Boxmoor Cricket Club before changing direction and going up steps back onto Station Road.

Police later revealed the man had also threatened his victim with a cutlery knife which was dropped when he fled.

The woman was not hurt in the attack but was left 'very shaken', police said.

Now officers investigating the incident have released this image of a rucksack with bright orange stripes they think is similar to one carried by the attacker.

He is described as a man of Eastern European origin in his mid 20s with olive skin and jet black hair, about 5ft 10ins tall and of medium build.

On the night of the attack he was also wearing a blue T-shirt and blue jeans.

A number of people were in the area when the incident occurred and police are appealing for anyone who recognises the offender to get in touch.

Anyone with information is asked to call Herts Police on 0845 33 00 222 or Crimestoppers anonymously on 0800 555111.