The widow of Russian opposition leader Alexei Navalny on Sunday night said Vladimir Putin 'must be held accountable' after it emerged her husband was killed by a frog poison.

Yulia Navalnaya thanked Britain and its allies for establishing that Mr Navalny's death two years ago is likely to have been caused by neurotoxin epibatidine.

The poison is only found in nature on the skin of the Ecuadorian dart frog and results in a painful death of paralysis and respiratory arrest if ingested.

Ms Navalnaya said: 'I was certain from the first day that my husband had been poisoned, but now there is proof: Putin killed Alexei with [a] chemical weapon.

'I am grateful to the European states for the meticulous work they carried out over two years and for uncovering the truth. Vladimir Putin is a murderer. He must be held accountable for all his crimes.'

On Saturday, the UK, along with Sweden, Germany, the Netherlands and France, accused the Russian state of killing 47-year-old Navalny in a Siberian colony.

Navalny had been sentenced to 19 years' imprisonment on dubious embezzlement charges. His death was announced by the Russian state on February 16, 2024. At the time, his allies accused the Kremlin of murdering him because of the political threat he posed.

In their joint statement, the five nations said laboratory analysis found traces of epibatidine in Navalny's body, a toxin associated with South American dart frogs.

The statement read: 'Epibatidine is a toxin found in poison dart frogs in South America. It is not found naturally in Russia.

'Russia claimed that Navalny died of natural causes. But given the toxicity of epibatidine and reported symptoms, poisoning was highly likely the cause of his death. Navalny died while held in prison, meaning Russia had the means, motive and opportunity to administer this poison to him.'

US Secretary of State Marco Rubio on Sunday described the report as Epibatidine, which is classed as a chemical weapon, is said to be 200 times stronger than morphine and can cause paralysis, respiratory failure and death.

It is used by indigenous South American tribes in blow darts or blowguns when they hunt.

It is not clear how the frog poison was allegedly administered to Navalny.

On Sunday morning, Foreign Secretary Yvette Cooper confirmed for the first time that Russia has had possession of the toxin, pointing out that 'it can be produced synthetically'.

Ms Cooper also said work has taken place over the last two years 'on pursuing the evidence and pursuing the truth' around Navalny's death. She said: 'They wanted to silence him because he was a critic of their regime.' 

Roman Abramovich has been warned 'time is running out' for him to hand over £2.5billion from the sale of Chelsea FC before it's taken out of his hands.

He agreed to transfer the cash – earmarked for humanitarian causes in Ukraine – after selling the Premier League club when he was sanctioned nearly four years ago following Putin's invasion. But the funds remain frozen in a bank account.

Mr Abramovich was told in December that ministers would take legal action if he failed to hand over money. 'The clock is ticking on him,' Yvette Cooper said on Sunday.