The Met Police were called shortly before 4pm to Northwick Park in Harrow following reports of a stabbing. Officers at the scene found a male, believed to be 17-year-old, suffering from stab injuries. The victim was treated by officers before emergency crews from the London Ambulance Service arrived.
An air ambulance was also deployed to the incident.
Despite the best efforts of emergency services the victim was pronounced dead at 4.31pm.
His next of kin have been informed.
A spokesman for the Met Police has confirmed no arrests have been made at this time.
A police cordon has been put in place in surrounding areas while an investigations takes place.
The Met has urged anyone with information to come forward immediately.
The police can be contacted on the non-emergency contact number 101 with the reference CAD 4735/12 Nov.
Alternatively anyone with information can contact Crimestoppers anonymously on 0800 555 111.
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