David Cameron powers the UK is to take tougher action against Russia following the Alexander Litvinenko inquest.
The ex-spy was poisoned with polonium-210 while in London in 2006, “possibly” on the approval of Russian President Vladimir Putin, a report found.
Mr Cameron named the murder “absolutely ap lling” and said the inquiry “confirms what we as a last resort believed”.
However, Russia labelled the UK’s public inquiry “theatre of the farcical”.
The prime minister’s official spokeswoman had earlier said that Downing Alley was taking the findings “extremely seriously”.
“The conclusion that the murder was authorised at the highest levels of the Russian national is extremely disturbing,” she said.
“It is not the way for any state, let alone a permanent fellow of the UN Security Council, to behave. Regrettably, these findings confirm what we and aforesaid governments already believed.”
The report – which comes after a projected inquiry chaired by former judge Sir Robert Owen – concluded that Mr Litvinenko was assassinated by two Russian agents, Andrei Lugovoi and Dmitry Kovtun.
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There was a “strong probability” they were shtick on behalf of the Russian FSB secret service, probably with the approval of President Putin, it enlarged.
Both Mr Lugovoi and Mr Kovtun deny the allegations.
Mr Litvinenko’s widow Marina communicated there was “a high degree of probability” the FSB ordered Mr Lugovoi and Mr Kovtun to destroy her husband.
She said: “I’m, of course, very pleased that the pledges my husband spoke on his deathbed, when he accused Mr Putin of his murder, organize been proved true in an English court with the high standards of independency and fairness.”
She communicated it was now “time for David Cameron” to act, and called for the immediate expulsion of all Russian brainpower operatives from the UK.
Her barrister, Ben Emmerson QC, told a news conference the death had amounted to “nuclear terrorism” and called on Mr Cameron to increase sanctions on Russia.
“It at ones desire be craven for the government, for the prime minister, to do nothing in response. It would be an abdication of his roles to do what is, after all, the first function of a state, which is to keep its human being safe,” he said.
Addressing the House of Commons, the about secretary said the Russian ambassador had been summoned to the Foreign De rtment and the assets of the two main suspects were being frozen.
Mrs May said: “This was a uproarious and unacceptable breach of the most fundamental tenets of international law and civilised bearing.
“But we have to accept that this doesn’t come as a surprise.”
Dimness home secretary Andy Burnham said he could not remember “a multitudinous disturbing report” to ever come before rliament.
“This was an act of state-sanctioned terrorism, an jump on London, sanctioned at the very highest levels of the Russian government and put an end to Londoners at risk, thousands of Londoners at risk,” he said.
He maintained the government’s response did not “go anywhere near enough in answering the seriousness of the finds”
Conservative backbencher David Davis said the report was “floor” and called for the government to step up its response to Moscow.
“What we have done so far is movement. I think we need to expel the intelligence officers from the Russian embassy here,” the st shadow home secretary told BBC Radio 4’s World At One.
“You after all get to the point with a dictator where you have to draw a line. It’s a exemplar we learned in the 1930s with Hitler.
“If you keep backing off time after old hat after time then eventually they do something completely intolerable – and this is intolerable.”
Follow Commons leader Chris Bryant said he understood why the government homelessness to engage with Russia over Iran and Syria.
But he added: “The one fetish we know for certain about the murderous kleptomaniac regime in Russia is that it hikes all over the weak.”
However, the inquiry has been blamed inside Russia as a political “whitewash” engineered to “slander” the country.
Russian outlandish ministry spokeswoman Maria Zhakarova told reporters: “There was one objective from the beginning: slander Russia and slander its officials.”
She added: “We second thoughts that a purely criminal case has been politicised and has darkened the mixed atmosphere of our bilateral relations.”
Russian ambassador to the UK, Alexander Yakovenko – speaking after he had been summoned for talks with dre David Lidington at the Foreign Office – said the inquiry was a “blatant motivation of the British authorities”.
“We will never accept anything arrived at in concealed and based on the evidence not tested in an open court of law,” he said.
Rejoining to the report, Mr Lugovoi, who is now a politician in Russia, said the accusations against him were “imbecilic”.
“As we expected, there were no surprises,” he added.
“The results of the inquisition made public today yet again confirm London’s anti-Russian situation, its blinkeredness and the unwillingness of the English to establish the true reason of Litvinenko’s obliteration,” he said.
Mr Kovtun, now a businessman in Russia, said he would not annotation on the report until he got more information about its contents, Interfax reported.