The footage starts with a guard dressed in uniform bringing a shackled prisoner, believed to Gao, into an office at about 4.20am. But the door to the prisoner’s cell is apparently left open, and soon afterwards two other prisoners – believed to be Wang and Li – are shown walking out of the cell door, without wearing chains or handcuffs.

One inmate is shown talking casually with the guard, but he suddenly lunges forward and strangles the officer. The other two prisoners rush into the room and appear to help kill the guard, who desperately tries to escape from the lock hold.

Shortly after the struggle, pne of the prisoners handcuffs the guard's wrists, while another is shown putting on the guard’s shoes. The three prisoners walking towards the prison gate, dressed in police jackets and shirts.

At about 4.40am all three prisoners are shown walking out the gate, with one of them casually smoking a cigarette.

The broadcast, on a popular news programme, has sparked intense debate on social media, with some people questioning whether it was ethical for CCTV to show images of the killing on national television.

Cheng Lei, an associate professor of criminal procedural law at Beijing’s Renmin University, told CCTV that the video footage showed there had been serious lapses in security, with guards not following rules set out by the Ministry of Public Security.

It was very unusual for a guard to bring a prisoner out of his cell during the night, Cheng said. Normally a prisoner could leave his or her cell only if there was an emergency, such as a riot, an attempted suicide a severe illness, but from the footage it appeared to be normal for Gao to be brought out, he said.

Also, no other guard had showed up during the 20 minutes it took for the prisoners to kill the guard and make their escape, even though other people should have been monitoring what was going on, he said.

It was also unclear how Gao had managed to remove his shackles, how the three prisoners had obtained police uniforms, and how they had managed to unlock a series of doors leading to the prison gates.

Cheng said that armed police officers should have been on guard outside the prison gates, but none was on duty at the time.

Beijing News reported that Gao was in prison after being found guilty of killing another villager. His death sentence had been under review by the Supreme People’s Court at the time of his escape.

Li was in prison after reportedly stabbing a man with knife. The victim was seriously injured, but his wounds were not life threatening.

Wang reportedly hired several other people to destroy the home of another man living in his village, in an early morning attack, following a dispute over land. An elderly woman was killed during the attack.

The provincial public security bureau had offered a reward of up to 100,000 yuan (HK$126,000) for information leading to the recapture of the three prisoners.

Escapes from mainland prisons are extremely rare. China imprisons a far smaller percentage of its population than the United States – 118 per 100,000 people compared with 737 per 100,000 in the US.

However, punishments are often harsh and China is believed to execute more people for crimes each year than the rest of the world combined.