In the latest attack, a 50-year-old woman walking to work through the underpass near Lady's Lane at about 9am on Saturday was approached from behind by a man who pulled at her skirt.

He ran off towards Wellington Street when she started screaming.

In an earlier incident last month, a 24-year-old woman pushing a buggy fought off a man who pulled at her clothing.

A Northamptonshire Police spokesman said: "This kind of incident is very unusual but we are doing everything we can to track down the people involved.

"We will be distributing posters around the town centre featuring a photo-fit of the offender in the first incident.

Young mum assaulted as she pushes buggy - click here

"In the latest incident the offender was waiting in the underpass so we do ask people to take extra care when using subways."

At the moment police are keeping an open mind on whether the two incidents are linked.

The offender on Saturday was described as of medium build, white, 5ft 8ins tall, dressed in a bright orange top and wearing three-quarter length beige trousers.

The earlier offender had been described as a light-skinned male, of stocky build, in his late 30s to early 40s and approximately 5ft 7in tall.

He had a round face and small eyes. He was wearing quite a lot of layers and a dark grey woollen beanie hat which covered his hair.

He was also wearing a dark fleece body warmer, a dirty jumper, which had dark red arms and possibly a white front with a motif on it and dark red trousers.

Police are appealing for witnesses to these incidents, or anyone who may have spotted anything suspicious, to come forward.

Officers can be contacted on 08453 700 700. Alternatively people can contact Crimestoppers anonymously on 0800 555 111.