Warning Graphic Content

Shocking video shows the moment a suspect being booked into a London jail suddenly whips out a hidden 128-year-old revolver — repeatedly opening fire and killing a veteran cop.

The harrowing footage from September 2020 was released Friday after Louis De Zoysa, 25, was found guilty of murdering Sgt. Matt Ratana, 54, who was just three months shy of retiring after nearly 30 years with London’s Metropolitan Police.

It starts with De Zoysa handcuffed in a chair in Croydon Custody Center, flanked by two other officers after being busted for possessing drugs and bullets.

“My name’s Matt — I’m the custody officer, right, I’m in charge,” Ratana informs him calmly after walking up, before asking him to “stand up” to be searched.

After a brief pause, De Zoysa gets to his feet — and immediately whips out an antique Colt .41, twisting his ‘cuffed hands to fire it in front of him.

The initial gunshot is met with an agonizing groan from Ratana, as the other two officers pounce on the killer, pinning him to the cell floor.

“Matt, are you all right?” one of the officers asks, repeating his name — just for the pinned gunman to fire three more shots while also being tased.

Ratana, who was originally from New Zealand, died after being hit in the chest, while another officer was hit in the thigh.

The killer also shit himself in the neck, leaving him wheelchair-bound and brain-damaged and needing a whiteboard to communicate.

It was only while giving the killer life-saving treatment from his own gunshot that officers realized he had a holster under his armpit where he had hidden the double-action revolver made in 1895.

De Zoysa’s lawyers argued he had pulled the trigger during an autistic meltdown. However, the court heard how he’d been forced to make his own ammo because none was available for the antique.

He was remanded until next month when he will be sentenced for murder.

Ratana’s girlfriend, Su Bushby, called it “a cruel end to a lifetime of service and dedication protecting others.”

“His life was taken too soon, in the line of duty, doing a job that he loved,” she said of the dad of one, whose son is also a cop.

“My love for Matt, my gentle giant, will never end. He will not be forgotten,” she said.

Met Commissioner Sir Mark Rowley also paid tribute to the “outstanding officer who brought joy to his work, treating everyone with respect, compassion and good humor.”

He also praised the “professionalism and bravery” of the other officers who pounced on the gunman.

“Without their courage I believe that more lives would have been lost,” the commissioner said.

The killing also had a “heavy impact” on the dead officer’s colleagues — including the top cop himself, he acknowledged.

“This tragedy has caused me to reflect on the uncertain world within which police officers operate and the risks they face day by day. Officers never have a perfect picture of what awaits them at the next incident,” he said.

“The men and women in policing, daily stepping forward into uncertainty and risk, are truly remarkable.”

Such shootings are rare in the UK, which has strict firearm laws. Ratana was the 17th from the London force to be killed by a gun since 1945.

Still, De Zoysa had purchased the antique revolver legally under a “obsolete caliber” exemption, police said — with the UK government then introducing new laws to ban that kind of weapon.

The Met also issued 4,300 hand-held metal detectors to front-line officers to search for weapons on first contact.

With Post wires