Louis De Zoysa was convicted of Sergeant Matt Ratana's murder in June - and it is believed he had concealed the antique revolver and holster under his armpit.

Northampton Crown Court was told that the 25-year-old deliberately fired the weapon without warning into the 54-year-old's chest, causing a fatal wound to his heart and lung.

De Zoysa was handcuffed at the time, and suffered brain damage after one of the shots fired hit him in the neck.

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CCTV footage suggests he managed to retrieve the weapon with his right hand about 16 minutes before the shooting - and used a vent at the back of his overcoat to hide the gun until the attack.

A total of four shots were fired within 20 seconds within a custody block in Croydon, south London, in the early hours of 25 September 2020.

Sgt Ratana had worked in policing for 30 years, and was close to retirement.