The death of Alexei Navalny must be “investigated fully” and those responsible “held to account”, Downing Street has said.
MPs are set to discuss the death of the jailed Russian opposition leader when they return to Parliament on Monday afternoon, as the Government weighs up its options for responding to the news.
Foreign Secretary Lord Cameron has already signalled that there could be fresh sanctions against Russian officials, amid questions for Russian authorities over how Mr Navalny died and a mounting chorus of Western voices holding Vladimir Putin responsible.
On Monday, Downing Street would not comment on possible future sanctions, but paid tribute to Mr Navalny and called for a full investigation.
Meanwhile, his widow Yulia Navalnaya claimed his family is being blocked from seeing his body as Russian authorities sought to conceal that he was killed using nerve agent Novichok, which he accused the Kremlin of using to poison him in 2020.
The Prime Minister’s official spokesman told reporters: “It is very clear that the Russian authorities saw him as a threat and that is why they imprisoned him on fabricated charges.
“The fact that the FSB (the Russian federal security service) poisoned him with a banned nerve agent and then sent him to an Arctic penal colony … his death must be fully investigated, and all of those in the Russian regime must be held to account.”
Foreign Office minister Leo Docherty is due to make a statement in the Commons on Mr Navalny’s death on Monday afternoon.
Ms Navalnaya said her husband was poisoned “by another of Putin’s Novichoks” as she criticised the authorities for stalling the release of the body.
After Mr Navalny’s mother was denied access to a morgue where his body was believed to be, Ms Navalnaya accused the authorities of trying to conceal evidence.
“They are cowardly and meanly hiding his body, refusing to give it to his mother and lying miserably while waiting for the trace of another of Putin’s Novichoks to disappear,” she said.
The Kremlin’s most prominent critic, the 47-year-old was imprisoned in January 2021 after he returned to Russia from Germany where he was recuperating from a near-lethal poisoning with a nerve agent.
He died in a penal colony on Friday, with Russia’s federal prison service saying he became unwell after a walk and lost consciousness.
It remains unclear what response the Government and other allies may take against Mr Putin, with Moscow already facing heavy sanctions since the start of the war in Ukraine.
At a briefing held by the think tank Chatham House, Mr Navalny’s biographer, Ben Noble, said the West needed to provide “more than thoughts and prayers” in response to the dissident’s death.
He said: “If the reaction is weak, then those political prisoners who are still alive in Russia will be in even more danger.”
He added: “We should make sure that the bite is as bad as the bark.
“If Biden is going to say there will be devastating consequences if Navalny were to die in detention, now he has of course died, then that has to be followed up otherwise it’s just another case of an empty threat from the West that Putin will regard as another sign that the West just doesn’t have the resolve to follow through on its promises.”
The row over Mr Navalny’s death comes as Ukraine and its allies prepare to mark the two-year anniversary of the Russian invasion.
Experts at Chatham House warned that, with American support for Kyiv in question, 2024 could be the toughest year yet for Ukraine.
Keir Giles, a senior consulting fellow at Chatham House, said shortages of munitions were having a “direct and immediate impact” on Ukraine’s ability to fight, and accused leaders in the UK and other Western nations of failing to take urgent action necessary to deter Russian aggression.
He said: “There is no evidence that the highest political level has understood the scale of the threat or tried to explain it to voters and the public who will have to approve the measures that need to be taken to avert it.
“If action comes too late to avoid disaster it will have been because of criminal complacency at the highest political level.”
It comes as Ukrainian president Volodymyr Zelensky struggles to convince Republicans in Washington to facilitate a major funding package for Kyiv.
Lord Cameron has urged lawmakers to pass the 60 billion US dollar (£47.6 billion) package.