A police officer has warned of the dangers of joining a gang and carrying a knife after a 13-year-old was locked up for killing an Edmonton man.

The teenager, who cannot be named for legal reasons, stabbed Jack Barry twice in the chest in the lift of flats in The Broadway, Edmonton, in December last year.

Detective Inspector Andrew Kelly described the incident as "tragic" and said the murderer was the youngest he has had to deal with during his career.

Speaking after the teenager was sentenced to 11 years behind bars, Det Insp Kelly said: "In my 11 years this is the youngest murder case we have dealt with.

"This is an appallingly tragic incident which demonstrates the dangers of carrying a knife. It shows how in an instant a minor argument over access to a communal door can escalate into a horrendous crime with long-lasting consequences for all the families involved.

"This event has robbed the family of Mr Barry of a much loved partner, and brother."

The stabbing was the third murder within the space of a mile in a month. Khiry Ford and Zbigniew Czarnocki both died on November 30.

Det Insp Kelly said: "The Metropolitan Police, led by its gang crime unit Trident, are actively discouraging people from joining gangs. This, as I said, is a perfect example of why you should not be in a gang, why you are misguided and why you should not carry a knife in public."