Sometime between 4.30pm and 5.30pm, the boy was cycling in Jubilee Park when he was approached by a group of teenage boys. They pushed him off his bike, threatened him with a spanner and stole his saddle and chain.

Police have issued descriptions of the teenagers.

The first is white, medium build with big shoulders, around 5ft 10ins tall, around 18 or 19 years old, clean shaven, wearing a diamond-style earring in his left ear, wearing a white v-neck top with sleeves to the elbow, navy blue tracksuit bottoms with three white stripes running down the legs and white trainers with Velcro straps.

The second assailant is described as being of mixed race, with a light complexion, slimmer build with big shoulders, around 5ft 7ins tall, around 18 or 19 years old and wearing a grey hooded top with the hood up, blue tracksuit bottoms and white trainers with Velcro straps.

The third is described as white, with a tanned complexion, skinny build, around 5ft 7ins tall, around 18 or 19 years old, clean shaven, wearing a grey hooded top with back wavy lines running vertically, black tracksuit bottoms and white trainers.

PC Ross George from the Targeted Patrol Team, said: "I would like to hear from anyone who recognises the descriptions of these three boys, or anyone who witnessed this robbery."

He added: "I am particularly keen to trace three girls, aged around 13 or 14, who were seen in the park shortly before the incident. Anyone who has any information can contact me in confidence."

PC George can be contacted at Alton Police Station on 0845 045 45 45 or call the Crimestoppers charity anonymously on 0800 555 111.