A spike in violent crime in London saw more murders committed in the city in February and March than there were in New York, figures show.
So far in 2018, the Met Police has investigated 46 murders, compared with 50 in the US city.
But, while New York's murder rate decreased from the end of January, London's rose markedly from that point.
Ex-Met Police Ch Supt Leroy Logan says it is proof that "London's violent traits have become a virus".
Statistics from the New York Police Department (NYPD) and the Metropolitan Police, reported in the Sunday Times and obtained by the BBC, highlight narrowing murder rates between the two cities, which have similar population sizes.
City Hall says it is "deeply concerned" by knife crime in the capital, but, along with the Met Police, insists London "remains one of the safest [cities] in the world".
- In January, the Met investigated eight murders whereas the NYPD looked into 18 killings
- By February, the NYPD's figures had dropped to 11, while London's rose to 15
- In March, 22 murders were investigated in London while 21 inquiries were launched in New York
- When the figures are weighted for population, the rate of murders per 100,000 people was higher in London than in New York in February and March.
The Met said it was "concerned at the increase in murders in London".
"One murder is one too many, and we are working hard with our partners to understand the increase and what we can all do to prevent these tragedies from happening in the first place," a spokesman said.
However, it is a murder rate that has left Mr Logan feeling "absolutely devastated".
"I cannot understand how things have gotten out of hand," he said.
"We have seen the virus of violence spreading. It is endemic in so many different parts of societies.
"It can only be dealt with in a holistic manner, because it is so holistic in its impact.
"Police can't just arrest or stop and search their way out of this problem; it has to be done in partnership with the communities."
The Met Police has launched 44 murder investigations in 2018, 31 of which have been as a result of stabbings.
The deaths of 47-year-old Laura Cecilia Navarrete De Figueira, from Twickenham, and her sons Claudio, 10, and Joaquin, seven, are part of the same Met Police murder investigation. She was found stabbed in London, while the boys were discovered dead, along with their father, at the foot of Birling Gap, in Sussex.
Croydon Central MP Sarah Jones, who chairs the all-party parliamentary group on knife crime, told the BBC's Today programme that London could learn from New York in how to reduce violent crime.
"New York has been able to bring down serious violence through a public health approach," she said.
"We need a proper strategy that looks at all of the issues.
"Knife crime and violent crime acts like an epidemic, so you need to go in at the source to cut it off and then you need to inoculate the future young people against it.
"Going in at source means major intervention work with youth workers, inoculating means going into schools, changing the social norms, educating kids, teaching them what it is to be a man, teaching them how they don't need to carry knives."
In February, the Met's Commissioner Cressida Dick visited police in Glasgow to learn about a public health approach which has seen murder rates in Scotland drop dramatically.
There are plans for Ms Dick to carry out more "fact-finding trips", in New York and with the West Midlands, Durham and Avon & Somerset forces.
Meanwhile, the government has launched a £1.35m campaign aimed at 10 to 21-year-olds.
The adverts to run across social media and digital channels feature stories of teenagers who have been stabbed.