A 17-year-old boy has been robbed at knifepoint while cycling along the canal towpath off St Mary's Road, Langley with two friends.

They were surrounded by a group of teenage boys between 3.45pm and 4.45pm last Thursday, who told them to empty their bags. The first offender produced a knife and took the victim's bike, a specialised Hard Rock mountain bike with a black and red frame.

The first offender is black, between 16 and 17 years old, around 5ft 10ins tall and had short black hair. He was wearing black jogging bottoms, a black sports t-shirt and what looked like a black dishcloth over his head.

The second and third offenders are both black, between 16 and 17 years old, and around 5ft 10ins tall. Both were wearing black jogging bottoms and a black sports top.

The fourth offender is white, between 16 and 17 years old, around 6ft 2ins tall and of a larger build. He had very short fair hair and was wearing a white t-shirt and white shorts.

Designated investigator Lynn Brandon, based at Slough police station, said: "This was a frightening experience for the victim and I am appealing to anyone who has any information to come forward."

You can contact police using an online reporting form, quoting reference number 43200160204. Or call the non-emergency number 101.

"Alternatively, you can call Crimestoppers anonymously on 0800 555 111."