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.

Jurors were shown distressing CCTV footage of the 26-year-old gun fanatic using the legally-bought gun to shoot Sgt Ratana.

Northampton Crown Court was told the former tax office data analyst deliberately fired the weapon without warning into the 54-year-old's chest, causing a fatal wound to his heart and lung. He later died in hospital

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

He had claimed diminished responsibility but was found guilty after a jury decided he pulled the antique weapon's trigger deliberately to fire homemade bullets, and had not suffered an autistic meltdown.

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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.

Sentencing De Zoysa, Mr Justice Jeremy Johnson told the 26-year-old: "You acted in cold blood. You intended to kill Sergeant Ratana.

"You aimed the gun at his chest at near point-blank range. Even as he fell you re-aimed and fired a second shot at him.

"The aggravating factors outweigh the mitigating factors. There is therefore no justification to depart from the starting point of a whole life order."

De Zoysa showed no obvious reaction as he was told he will spend the rest of his life in prison.

Sgt Ratana's partner Su Bushby said in an impact statement: "Losing Matt, my partner for five years, is the most devastating event that has happened to me in my life.

"Not a day goes by, when I don't think of him; miss his smile, his humour, his laughter and his wonderful kind personality.

She added: "Whilst the court proceedings will never bring my Matt home to me, knowing that his cowardly killer has been brought to justice makes me feel that there is some closure on the events of that terrible morning of the 25th of September 2020."

Other statements from Sgt Ratana's adult son Luke and his mum Theresa who live in Australia, as well as representatives of his family in New Zealand, were also read out.

Met Commissioner Sir Mark Rowley spoke outside the court saying: "Matt dedicated almost 30 years to policing and was nearing retirement when he was tragically murdered. He was an outstanding officer who brought joy to his work, treating everyone with respect, compassion and good humour.

"In the days after his death, tributes flowed in from Matt's colleagues, from communities he had served and from those who knew him in his life outside policing. They were a testament to the man he was."