A cannabis user who shot dead a custody sergeant while handcuffed in a police cell has been handed a whole-life sentence.

Gun fanatic Louis De Zoysa was convicted of murder last month, after jurors were shown distressing CCTV footage of the 26-year-old using a legally-bought revolver to gun down Sergeant Matt Ratana.

Sgt Ratana died in hospital despite the efforts of medical staff, after being struck by two bullets in a holding cell in Croydon, south London, in September 2020.

The jury which convicted De Zoysa, a former tax office data analyst, was not told that a shortened infantry rifle, numerous types of ammunition, a pipe gun and a dummy launcher were found at his rented property after the killing.

Sentencing De Zoysa at Northampton Crown Court on Thursday, High Court judge Mr Justice Jeremy Johnson told him: “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, who is expected to be held at HMP Belmarsh in south-east London, showed no obvious reaction as he was told he will spend the rest of his life in prison.

De Zoysa, of Banstead, Surrey, claimed diminished responsibility but was found guilty after the jury decided he pulled the antique weapon’s trigger deliberately to fire homemade bullets, and had not suffered an autistic meltdown.

His three-week trial was shown slow-motion video of New Zealand-born Sgt Ratana being hit in the chest by the first of three shots discharged within three seconds.

A second bullet struck the 54-year-old in the thigh before De Zoysa was wrestled to the ground by other officers, as a third round hit the cell wall at Croydon’s Windmill Road custody centre.

De Zoysa discharged a fourth shot while on the cell floor 16 seconds later, hitting an artery in his own neck and causing brain damage.

The judge said victim impact statements made to the court by the siblings, son, stepmother, former partner and partner of Sgt Ratana showed they were “rightly immensely proud” of him.

Stressing that the sentence imposed on De Zoysa was one “of last resort” reserved for cases of the most extreme gravity, the judge told him: “A whole life order means that you must remain in custody for the rest of your life.

“The events are clear on CCTV and body-worn video evidence. You knew the gun worked, you knew that each cartridge was likely to kill if fired directly at a person’s chest.

“You deliberately concealed the gun from the police. You told them you had cannabis to focus their attention on that.

“You made strenuous and successful efforts to retrieve the gun while you were handcuffed behind your back.”

De Zoysa, who is thought to have removed the gun from a holster while in a police van and then kept it hidden under the vent of his overcoat, had time and space to consider what to do, the judge said.

“You were able to think about what you would do,” he continued. “Autism is not to blame for your decision to murder Sergeant Ratana. You are to blame for that.”

The court was told that officials at Belmarsh have confirmed the prison has the resources and facilities to provide the type of medical care De Zoysa requires.

Before De Zoysa was sentenced, the court heard victim impact statements from Sgt Ratana’s partner Su Bushby, his younger sister Jessica Williams, younger brother James Young, his former partner Theresa Ratana, his son Luke Ratana, and his stepmother Diane Peachey.

Ms Bushby told the court: “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 also described the pain of being told of her partner’s death, the “indescribable” impact of seeing his body in hospital, and the “horrendous” experience of seeing CCTV footage of his “cruel and cowardly” murder.

Matt understood the risks of working as a police officer, but being killed in such a way, in his own police station, is just too horrible for me to bear

Reading her statement from the witness box, Ms Bushby said of the CCTV evidence: “It was so difficult to hear Matt’s voice and see him in the video being a custody sergeant, a role he loved, knowing, when he walked into that room, his life would be over a few seconds later.

“I have so much praise for the swift actions of the officers and staff that were in the custody area that night.

“Matt understood the risks of working as a police officer, but being killed in such a way, in his own police station, is just too horrible for me to bear.”

Offering mitigation and arguing against the imposition of a whole life order, Imran Khan KC said De Zoysa had experienced a troubled childhood and was proud of his work as an analyst.

“He had the potential of a good future ahead of him,” Mr Khan said. “We may never know the reason why he had a loaded revolver on him.”