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While the formal cause of Mr Petryszyn's death has not been confirmed, tests showed he had "spice" in his system - a lab-made drug that mimics the effects of cannabis and is illegal.

“If this is true, then no matter the formal cause the responsibility for the premature death is Vladimir Putin personally, who first gave the green light to the poisoning of Alexei and then put him in prison,” said Mikhail Khodorkovsky, an exiled Russia tycoon turned opposition figure in exile, speaking in an online statement.

A post-mortem examination was carried out on Thursday but no formal cause of death was established, with further tests due to be carried out.

A pathologist had given the formal cause as unascertained due to the state of decomposition of the body.

Jurors at the Royal Courts of Justice heard the formal cause of Amman's death was shock and haemorrhage due to gunshot wounds to the neck and abdomen.