Standing in his best Nike T-shirt, arms folded and mouth beaming towards the camera in a cocky grin, the image could be any one of thousands uploaded to social media by teenagers every day.

But experts believe that for this 16-year-old, imprisoned after plunging a hunting knife into a rival gang member’s chest, it is a means of pursuing territorial disputes and maintaining loyalties beyond the walls of his prison.

Youth workers and criminologists have warned that teenage offenders’ rehabilitation is being undermined by the use of smuggled smartphones, which allow access to accounts on which gangs taunt each other by flaunting weapons and bragging about encroaching on rival territory.

The inmate is a member of a gang based around Caledonian Road – a stretch of shops and terraced homes that runs north from London’s King’s Cross. After his conviction for attempted murder in November, the Crown Prosecution Service said: “A dangerous gang member will now spend a significant amount of time in prison where he will no longer be a risk to the public.” Weeks into his sentence, he posted the image with the caption: “14 years and still smiling.”

Many of his 1,000 Instagram followers herald him as a “savage” who strikes fear into the hearts of their “opps” – or opposition. One has recently uploaded CCTV footage of the attack, which was released to the media after conviction, with the caption: “Somebody ask him if that one healed. #TheRealChestShotGang.”

Gang members in Caledonian Road frequently clash with opponents in neighbouring areas, including “Easy Cash” in the EC1 postcode; “Red Pitch”, whose territory runs behind a strip of bars in Dalston; and Hoxton, or “Hox”, just north of the UK’s tech startup hub known as Silicon Roundabout. Their rivalry is believed to have accounted for at least four deaths and a string of near-fatal knife attacks within little more than three square miles since 2015.

Among the 16-year-old’s followers is a teen who recently posted an image from a rival gang’s area, with the caption: “Opp block [rival’s area] everyday EC [Easy Cash] a fucking joke.” A 15-year-old from EC1 responded: “Be posted on ur block coz when we came we didn’t see no one only civil [civilians].” Another follower posted six times from HMP Chelmsford last week, including a selfie with the caption: “Smartphones all over the jail.”

The concerns at the potential for social media to provoke violent crime follows a rise in fatal stabbings in London, where there have been at least 46 knife deaths this year. Craig Pinkney, a lecturer in criminology at University College Birmingham who was one of the first in the UK to study gangs’ use of social media, believes it is “fuelling and inciting a lot of violence”. But he added that social inequalities, such as poverty and inadequate education, were the main drivers. The criminal justice system focuses too little on rehabilitation, he said, warning that inmates’ unfettered use of social media would reduce their already slim chances of breaking the cycle. “If you are still able to access social media, psychologically you are still connected,” he said.

There are also fears that social media access is thwarting attempts to break the cycle of violence. “The young people are maintaining that gang facade. They’re not detached and that makes our job impossible,” said one youth worker based in a prison near London.

Retired Metropolitan police superintendent Leroy Logan, who chairs Voyage Youth, a charity working to empower marginalised young men, said minimising reoffending was key to reducing deaths. He fears that inmates’ access to smartphones complicates this task. “You have got to change their habits, behaviour and contacts. If they’re still running with the same people, albeit on a mobile phone, you reduce the possibility of them being rehabilitated. They are still immersed in the thug life.”

But some human rights groups are uneasy about drawing links between online habits and violence. Social media monitoring by the Met has been criticised by Amnesty International, as it risks “profiling” young people “simply because of the subculture to which they belong and the people with whom they associate online”.

Snapchat and Instagram said they encourage users to report inappropriate content, which they will investigate and remove, as appropriate. Snapchat said that it was a private messaging service and, as such, did not actively monitor posts. It encouraged anyone who sees inappropriate content to report it so it can be investigated by its “trust and safety team”. Messages are automatically deleted after 24 hours or less, it said.

Instagram said it does not tolerate criminality, including posts that support or praise a criminal act, on its platform. It said that users could report troubling material, which would be quickly reviewed and removed if found to breach guidelines.

It is illegal for prisoners to use mobiles phones; last year 15,000 sim cards were confiscated in England and Wales, mostly in adult prisons, where authorities believe there is a greater risk of inmates controlling outside criminal activities. Glyn Travis, of the Prison Officers’ Association, said smartphone use had reached “epidemic proportions”. “The crisis is preventable but no one listens,” he said. Andrew Neilson, director of campaigns at the Howard League for Penal Reform, said that a “failure to provide positive alternatives” led inmates, who are often locked in their cells 23 hours a day, to seek smuggled phones.

A Prisons Service spokesperson said: “Illegal use of mobile phones is a much less significant problem in youth offender institutions than in prisons – only 66 mobile phones were confiscated on the youth estate last year. We nonetheless take the issue very seriously and are investing £2m to prevent mobile phone use. Anyone caught using mobiles faces further punishment.”