Children as young as five should be taught about the dangers of knife crime, a police chief has said, as figures revealed the death toll among under-25s in London had almost doubled year-on-year.

DCS Sean Yates, Scotland Yard’s head of knife crime, said waiting until children were older risked leaving their initial exposure to scenes of violence to social media.

Official figures show that 2017 was the worst year for knife deaths among young people since at least 2002. Forty-six people aged 25 or under were stabbed to death in London, up by 21 compared with the previous year, according to police figures.

Yates said social media was an increasing factor in stoking grudges between youngsters that led to knife attacks. He also told the Guardian that courts were failing to enforce a “two strikes” law aimed at jailing those caught carrying a knife on two occasions, which was frustrating law enforcement.

The Metropolitan police has said knife crime should be treated as a public health issue, and relying on arrest and law enforcement tactics will not quell the menace it poses.

“If youngsters are going to go out and stab each other it is very difficult to police,” Yates said. “We need to be talking to these youngsters at a very early age. Why wait until they are exposed to social media.”

Yates said messages about knife crime should reach children before they are at secondary school. “The teachable moment needs to be before they are lying in A&E,. We are not going to arrest and enforce our way out of this. It is a public health issue.”

The big rise in the number of young people killed by knife crime in London came despite the Met increasing its use of suppression and enforcement tactics, carrying out more stop and search, more weapons sweeps and other activity on the streets of the capital. The Met commissioner, Cressida Dick, has declared that reversing rising violent crime is a priority.

Yates said knife crime deaths would have risen even more without the increased police activity. In 2017, knife crime was responsible for 20 out of 26 teenage homicides in London, and the capital responsible for most of the 39 teenagers stabbed to death in England and Wales.

This year has already seen three stabbing deaths of people aged 25 or under in London. The latest, on Friday in Shepherd’s Bush, west London, claimed the life of a model, Harry Uzoka, 25, who was stabbed in broad daylight.

In another incident in London, a 23-year-old was found stabbed in the abdomen just after 6.30am on Sunday on Edgware Road. They are expected to survive.

Outside the capital, a 19-year-old died in Walsall, in the West Midlands, after being stabbed just before 2am on Sunday at a house party.

The attention of the media and politicians was caught by a spate of four knife killings on and around New Year’s Eve.

Yates said grudges were escalating via social media, with perpetrators sometimes feeling disrespected and seeking revenge. He said: “Before, it would stop at the school gates. It becomes viral around the school, it becomes a respect issue, then they taunt each other.When we deal with these incidents we are seeing social media on phones, more and more.”

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He added that the “two-strikes” knife law, introduced in 2015, was not being implemented often enough by courts, weakening its deterrent effect.

He said: “It is not always enforced. The police are putting people before the court. Not in all cases are you seeing the two-strikes law implemented by the judiciary. What message does it send out? We need to have support from the judiciary.”

There has been debate among those concerned with tackling knife crime about whether there has been a cultural change among young people in terms of a willingness to carry and use bladed weapons.

Yates said children were “increasingly arming themselves” with a knife and that police had seen an increase. “It has become more socially acceptable in their groups. Is it because they are more at risk? Do they carry a knife to protect themselves, to intimidate to protect their (drug) supply lines, or carry out a robbery?”

Austerity and public spending cuts have led to a reduction in places where young people have somewhere to go and not be left out on the streets.

Budgets cuts to policing mean there are fewer Met officers patrolling the capital’s streets as its population and violent crime levels rise. Yates said: “It is challenging, we are cutting our cloth accordingly. There are less cops that are visible and out on the streets.”

Yates said police had community support for increased stop and search, with the tactic being controversial because it is used disproportionately against black people, with the vast majority of searches not detecting any crime.

Yates said the proportion of stops where the officer gave the reason for using the tactic as suspecting someone may have a weapon had risen from 12% to 18%. Most stops were still for drugs.

Duwayne Brooks, the surviving victim of the knife attack that killed Stephen Lawrence in 1993, whose case convulsed the criminal justice system, said stop and search was being misused and does not work.