Alexei Navalny was likely killed by a single punch to the heart, a technique used by the KGB, a source inside the prison where the Putin critic was being held, has reportedly claimed.

Bruising found on the opposition leader’s body was consistent with the ‘one-punch’ execution method, according to Russian exile and human rights campaigner Vladimir Osechkin.

‘It is an old method of the KGB’s special forces divisions,’ he told The Times. ‘They trained their operatives to kill a man with one punch in the heart, in the centre of the body. It was a hallmark of the KGB.’ 

Osechkin is not convinced and says that authorities would have been able to kill Navalny in any way they desired, and would not have wanted to ‘leave a trace in his body and would lead directly back to Putin’.

He claims that Navalny had been forced to spend between two and a half to four hours in an open-air solitary confinement space where temperatures could dip to minus 27C the day before his death.

Prisoners are normally kept outdoors for no more than an hour and in far less extreme conditions.

‘I think that they first destroyed his body by keeping him out in the cold for a long time and slowing the blood circulation down to a minimum,’ Osechkin explained.

‘And then it becomes very easy to kill someone, within seconds, if the operative has some experience in this.’

The single punch, a stealth assassination technique reputedly used by KGB special forces to avoid leaving any indication of a cause of death, is then said to have been used.

Before becoming Russian leader, Putin famously served for some 15 years in the KGB as a foreign intelligence officer, retiring in 1990 with the rank of lieutenant colonel.

Russian authorities have so far failed to give a convincing explanation for the death of Navalny, the Russian president’s most prominent critic.

According to the National Penitentiary Service, the 47-year-old died after taking a walk and feeling unwell.

A state-controlled channel on the Telegram messaging site later claimed the cause of death was a blood clot, however.

Mr Navalny’s family has been told that they cannot have access to his body for another two weeks.

Osechkin said that he believes the presence of FSB officers at the prison to be evidence that Mr Navalny was murdered by the Kremlin.

He added: ‘From what I know from my sources, it was a special operation that had been prepared several days in advance.’

‘It was a command from Moscow because without Moscow it would not have been possible to dismantle the cameras in the way that they did.’