A third person has been arrested in connection with the fatal stabbing of aspiring lawyer Sven Badzak in north-west London.

The 22-year-old was knifed to death in a “completely unprovoked” attack involving six people as he went to buy a bagel in Kilburn on February 6, the Metropolitan Police said.

A 20-year-old man was arrested on suspicion of murder on Sunday morning and remains in custody at a north London police station.

A 19-year-old man and a 17-year-old boy previously arrested on suspicion of murder have been released on bail.

Mr Badzak, who was working for his father’s construction company and hoped to do a law conversion course, was waiting outside a bagel shop with his 16-year-old friend when they were both attacked.

His mother, Jasna Badzak, a former Conservative Party activist, said she believes her son was killed in a random attack.

Detectives do not think the victims knew their attackers and have no information to suggest it was a robbery.

Ms Badzak said her son, who was privately educated at Wetherby and Portland Place schools before attending Roehampton University, had gone to a shop to get orange juice and then to a bagel shop when he was attacked.

As he and his friend were chased at around 5.30pm, they became separated and Mr Badzak, from Maida Hill, west London, fell to the ground and was stabbed repeatedly.

The 16-year-old also suffered multiple stab injuries but managed to escape into a shop in Willesden Lane.

He is in a stable condition in hospital.

Ms Badzak said her son, who had a degree in sociology but wanted to become a lawyer, had met Boris Johnson and other well-known Conservative politicians when he was younger through her campaigning work with the party.

The Prime Minister said “absolutely everything” is being done to tackle violent crime after Ms Badzak, a consultant and financial analyst, called on him to help ensure her son’s killers are caught.

Anybody with information is urged to call police on 101 or tweet @MetCC quoting CAD 5580/06FEB.

Alternatively, information can be provided to Crimestoppers, anonymously, by calling 0800 555 111.