A 16-year-old girl was hit on the back of the head and robbed of her mobile phone by a man on a bike.
Police are now appealing for witnesses to the robbery which happened at around 11.15am last Friday.
The girl was walking through the underpass on the redway, towards the H5, when she was approached by a man on a bike.
The girl then heard a smash and felt something on the back of her head.
She fell to the floor and the man demanded her Samsung Galaxy S3 mobile phone, which she handed over.
The offender then rode off on a silver bike in the direction of Grange Farm.
The offender is described as white, around 5ft 8ins tall, slim, and between 20 and 25-years-old. He had dark eyebrows and facial hair and was wearing a royal blue hooded top under a black jacket.
The girl suffered cuts and bruises but did not require hospital treatment.
If you have any information contact detective constable Wendy Cissell from Milton Keynes police station via the Thames Valley Police Enquiry Centre on 101.