A young man is the second to be fatally stabbed on the streets of the capital within 24 hours.
The 19-year-old was found suffering stab wounds by paramedics from London Ambulance Service in a cul-de-sac in Northolt, north-west London, around 3.20pm on Tuesday.
He was rushed to hospital from the scene in Newnham Close where he was pronounced dead just after 5.35pm.
Four men have been arrested on suspicion of murder and are currently being held at police stations across London.
Scotland Yard confirmed the victim’s next of kin had been contacted.
A spokesman added: “The Homicide and Major Crime Command has been informed and will be investigating.”
The victim is the second young man to be stabbed to death in the capital in the space of just 24 hours.
On Monday, a man, thought to be in his 20s, was fatally knifed in Plumstead, south-east London.
Witnesses said the victim's family "screamed with grief" after the attack.
Police said they were called with paramedics to Bournewood Road, a quiet suburban cul-de-sac, at about 4.30pm and found the man in the street suffering fatal stab wounds.
The air ambulance was scrambled but medics were unable to save the man.
Resident Caroline Juby said: “There were police everywhere. There were some youths at the scene in distress and the family of the boy. They were screaming with grief, it was horrible to see and hear.”
Witnesses are asked to contact Ealing police on 101 or tweet @MetCC.
Alternatively, contact Crimestoppers anonymously on 0800 555 111 or online at crimestoppers-uk.org.