A 17-year-old boy received a severe and "potentially life-changing" stab wound to his arm in a fight with an unknown man and woman near West Hampstead underground station .

Police believe the victim left the station at 5.54pm on Sunday (October 28), followed by a man and woman who fought him on West End Lane for as yet unknown reasons before re-entering the station without paying.

They are searching for a female witness who approached officers at the crime scene or anyone else with information related to the incident or those involved.

Detective Constable Rob Griffiths said: "This is a violent attack in which the victim has received very serious and potentially life-changing injuries.

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"The motive for this attack is not yet known, although we have no evidence to suggest it is gang-related.

"We do not know if the victim and suspects were known to each other beforehand, or they met just prior to the attack.

"I am appealing for anyone with information to get in touch with police and provide us with any details they may have, however insignificant they might seem.

"We would particularly like to speak to a female witness, who we believe approached officers at the crime scene originally and could have vital information that might help progress our investigation.

"If you are this woman, we would ask you to please contact us again as soon as possible."

As yet, there have been no arrests.

Anyone with information is asked to contact police on 101 and quote CAD 6012/28Oct or call Crimestoppers anonymously on 0800 555 111.