Two teenagers are among four people to have been stabbed to death in a series of apparently unrelated attacks across London within a 24-hour period.
Scotland Yard confirmed the male victims - a 17-year-old, 18-year-old and a 20-year-old on New Year’s Eve, and a 20-year-old in the early hours of Monday - all died as a result of their injuries, while a fifth is critically ill in hospital.
Five men are in police custody in connection with one of the deaths, while forensics officers continue to spend the start of 2018 behind cordons scattered across the city.
Met Commander Neil Jerome said four deaths in such a short period was unusual.
He said: "There were small groups who chose to engage in disorder and violence, and whose actions have had utterly tragic consequences.
"It is heart-breaking that, at a time when so many of us are contemplating what lies ahead in 2018, four families are dealing with the grief of losing a loved one to senseless violence and the callous use of knives as lethal weapons.
"I can assure Londoners - and the families and friends of the four males killed and the fifth who remains in hospital - that detectives will work tirelessly to bring to justice those responsible for such despicable acts of violence.
"As we begin these investigations and the provision of support to four grieving families, days like this serve to reinforce the determination of the Metropolitan Police to tackle knife crime and violence on our streets; especially offences committed by and against young people.
"I would urge anyone with information about these attacks, and more generally about anyone who routinely carries a knife, to urgently pass that information to police or Crimestoppers.
"If you’ve hesitated to do so before, then these four tragic deaths at the start of a new year may persuade you to act now to make London safer for your family and friends in 2018."
Sunday’s stabbings - at 11.30am in Larmans Road, Enfield, 7.35pm at Memorial Avenue in West Ham, and 10.40pm in Norwood Road in Tulse Hill - took the number of people stabbed to death in the capital last year to 80, Scotland Yard confirmed. Of those, 18 involved victims under the age of 19.
The New Year’s Day stabbing occurred at Bartholomew Court, Old Street, in the City.
The latest deaths will be seen as a setback to London mayor Sadiq Khan and his aim to get a stranglehold on the year-on-year increase in knife crime in the capital, which has seen the launch of a high-profile "London needs you alive" campaign and knife crime strategy.
At his last Mayor’s Question Time, Mr Khan said: "Every death on the streets of London is an utter tragedy and I’m deeply concerned about the rise in knife crime.
"I am supporting the delivery of community-led solutions and delivering interventions at critical points to offer real opportunities to exit offending behaviour as well as coming down hard on those who habitually carry knives."
In a statement following the latest stabbings, Mr Khan renewed police appeals for information.
He said: "We will work tirelessly in 2018 and beyond to stamp out this scourge.
"I want to be absolutely clear - if you use a knife the full force of the law will be brought down on you. You will be caught and prosecuted."