The wife of the late Russian opposition leader Alexei Navalny has said he was killed by poisoning while serving a prison sentence in an Arctic penal colony in 2024.

In a video shared on social media, Yulia Navalnaya said analysis of smuggled biological samples carried out by laboratories in two countries showed that her husband had been “murdered”.

She did not provide details on the poison allegedly used, on the samples or on the analysis – but challenged the two laboratories to publish their results.

Navalny – an anti-corruption campaigner and Russia’s most vociferous opposition leader – died suddenly in jail on 16 February 2024 at the age of 47.

In 2020 he was poisoned with a Novichok nerve agent and almost died. He underwent treatment in Germany, and was arrested at the airport upon his return to Russia.

At the time of his death he had been in jail for three years on trumped-up charges and had recently been transferred to a penal colony in the Arctic Circle.

Navalny’s supporters and colleagues at his Anti-Corruption Foundation (FBK) have always maintained the Russian government was involved in his demise.

Navalnaya said that after her husband’s death in February 2024 his team were able to “obtain and securely transfer” biological samples abroad and that two laboratories in different countries had concluded he had been poisoned.

She did not share the location of the laboratories – but she implied that they were not making their findings public due to “political considerations”.

“They don’t want an inconvenient truth to surface at the wrong time,” she said.

Navalnaya also suggested she would get pushback on trying to investigate her husband’s death further: “‘You are the wife, of course, but there is no criminal case, there are no legal grounds to hand documents to you.'”

“But I have grounds. Not legal, but moral grounds.”

She added that Navalny had been her husband, friend and closest person – and “a symbol of hope for a better future for our country”.

“I know he was a symbol to you too,” she said over images of Navalny’s Moscow funeral which drew thousands despite warnings from the authorities not to attend.

“I will not be silent. I affirm that Vladimir Putin is guilty of killing my husband, Alexei Navalny… I urge the laboratories which conducted studies to make the results public.”

On Wednesday Kremlin spokesman Dmitry Peskov said he was unaware of Navalnaya’s statements.

In the video, Navalnaya also detailed her husband’s last days based on what she said was testimony by employees at the penal colony, which the BBC has not been able to verify.

According to her, on the day he died Navalny was taken out for a walk but felt ill. When he was taken back to his cell “he lay down on the floor, pulled his knees up, and started moaning in pain… then he started vomiting”.

“Alexei was having convulsions… the prison guards watched [his] agony through the bars of the cell window,” she said, citing the alleged testimonies.

An ambulance wasn’t called until 40 minutes after Navalny became ill, his widow said, and he died shortly after. Prison authorities later told his mother Lyudmila that her son had experienced “sudden death syndrome”. Later, state investigators said the death had been caused by a medical condition and arrythmia.

Navalny’s associates have shared previously unseen images on social media purporting to show his cell on the day he died and the tiny exercise yard where he was allowed out.

Vladimir Putin, who studiously avoided naming Navalny while he was alive, briefly referred to him a month after his death by stating that a person passing was “always a sad event”.

The Russian president also said he had agreed to a planned prisoner swap between Navalny and “some people” held in Western jails, on condition that Navalny did not come back to Russia.

“But such is life. There’s nothing to be done about it,” Putin said.

It is highly unlikely Moscow will issue any further comment on Navalny’s death.

His popularity and internet savviness long rattled the Kremlin, while senior figures were irritated by his investigations into high-profile government corruption.

With Navalny’s death Russia lost the last towering opposition figure who challenged Putin’s rule.

Many of his associates have been jailed or have fled Russia. Navalnaya herself faces arrest, and she and her two children live abroad.

The crackdown on civil society ramped up further following Russia’s full-scale invasion of Ukraine in February 2022, and punitive new laws leading to mass arrests have muzzled any opposition.

In both life and death Navalny managed to draw out huge crowds onto the streets. Thousands of mourners turned out for his funeral in Moscow in March 2024 despite well-founded fears of a police crackdown.

No large opposition gatherings have taken place in Russia since.