She said: “We will never tolerate a threat to the life of British citizens and others on British soil from the Russian government.”
Her comments come as Moscow announced tit-for-tat expulsions of 23 British diplomats, the closure of the British consulate in St Petersburg and the shutting of the British Council, which promotes cultural links between the two nations.
Mrs May was applauded at the Conservative Party Spring Forum in London as she stood in solidarity with the people of Salisbury.
She praised the bravery of emergency services who helped Russian double agent Sergei Skripal and his daughter Yulia after they were poisoned with Novichok, a military- grade nerve agent developed by the Soviet Union during the Cold War.
Theresa May vows not to cave in to Kremlin threats
We will never tolerate a threat to the life of British citizens and others on British soil from the Russian government
Saying there was “no alternative conclusion other than that the Russian state was culpable”, Mrs May described the “appalling” attack as a “flagrant breach of international law”. She added: “We will consider our next steps in the coming days, alongside our allies and partners”.
But the Prime Minister pointedly omitted Labour leader Jeremy Corbyn as she listed those who backed her tough stance.
“We can be reassured by the strong support we have received from our friends and allies around the world,” she said.
“From the United States, Nato and the European Union. From our UN and Commonwealth partners. I am grateful too from the strong support I have received from the First Ministers of Scotland and Wales.
“And in the House of Commons this week, we saw a consensus as member after member, across all parties, stood up to condemn Russia’s actions and to support the position of Her Majesty’s Government.”
Mr Corbyn’s own frontbenchers broke ranks last week after he repeatedly failed to unequivocally condemn Russia for the attack and suggested Britain should send a sample of the nerve agent to the Kremlin.
Mrs May added: “Because this act of Russian aggression is the very antithesis of the liberal and democratic values that define the United Kingdom. The rule of law. Freedom of speech. The toleration of dissenting and minority views. A free press. Fair and democratic elections. A thriving civil society.
“These are the foundation stones of human freedom. They are the values which unite us as a country and they must always be defended.”
The Foreign Office said yesterday: “Our ambassador in Moscow has just been informed by the Russian government that they will expel 23 diplomats, close the British Consulate- General in St Petersburg and close the British Council in Moscow.
“This follows the action we have taken, alongside other measures, to dismantle the Russian espionage network operating in the UK as a consequence of the attempted assassination of two people here in Britain using a nerve agent.
“In light of Russia’s previous behaviour, we anticipated a response of this kind and the National Security Council will meet early next week to consider next steps. Our priority today is looking after our staff in Russia and assisting those that will return to the UK.
Corbyn’s own frontbenchers broke ranks last week after he repeatedly failed to condemn Russia
“Russia’s response doesn’t change the facts of the matter – the attempted assassination of two people on British soil, for which there is no alternative conclusion other than that the Russian state was culpable.
“It is Russia that is in flagrant breach of international law and the Chemical Weapons Convention.
“We have no disagreement with the people of Russia and we continue to believe it is not in our national interest to break off all dialogue between our countries.
“But the onus remains on the Russian state to account for their actions and to comply with their international obligations.”
Britain has already invited the Organisation for the Prohibition of Chemical Weapons (OCPW) to send inspectors to examine the nerve agent, thought to be Russian-produced Novichok, used against Mr Skripal, 66, and Yulia, 33, two weeks ago.
The Government is expected to explore further steps at both a National Security Council meeting and EU summit next week.
One move could see the revocation of travel visas belonging to Russia’s top oligarchs and politicians and their families, preventing the use of Brit- ain as a luxury shopping and education destination.
On Wednesday, Russian opposition leader Alexei Navalny – who was banned from taking part in today’s general elections – said British financial sanctions currently aimed at Russian diplomats and politicians should be broadened to include billionaire oligarchs who have close relationships with Vladimir Putin.
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Those in the ring line could include tycoons Roman Abramovich, owner of Chelsea FC, Alisher Usmanov, who owns Arsenal and Igor Shuvalov, Russia’s wealthy deputy prime minster, said to own two White- hall properties worth £11million, despite declared assets of £650,000.
Others could include Oleg Deripaska, ex-owner of energy conglomerate En+ which floated on the FTSE last year, and brothers Boris and Arkady Rotenberg, President Putin’s friends for 40 years and owners of the SGM group, Russia’s largest construction company for gas pipelines and electrical power supply lines.
Kremlin opponent Dr Yuri Felshtinsky, the author of a new book on Russia, The Putin Corporation and who also co-wrote a book with Alexander Litvinenko, the spy murdered by polonium poisoning, said: “Putin’s lesson after the murder of Litvinenko in 2006, the invasion of Georgia in 2008 and the annexation of Crimea in 2014 is that there is no punishment.
“Even sanctions over the Ukraine were minimal for an act that killed 12,000 people. The way to really hit hard is to target the top 1,000 oligarchs and members of parliament and their families with travel bans.
“We shouldn’t just take property or money in Britain from them – the most effective answer is to take the whole of Britain away from them.”
He said most oligarchs have escape plans such as second citizenships because, “ultimately, they don’t trust the Russian government. That’s why they buy property abroad; that’s why they send their children to schools in the UK.
‘It is an easy matter to make someone persona non grata – it is well within Britain’s rights. “
Dr Felshtinsky said it was no coincidence that the attack happened in run-up to Russian elections
He said the list would begin with the 100 closest allies and extend to 1,000, including all members of certain parties in Russian’s parliament.
“Russian’s parliament has little to do with democracy – it is full of Putin supporters. Imagine if you revoke the travel visas for all members of certain political parties and their families. You would have wives crying to their husbands because they would no longer be able to shop abroad, or send their children to school.
“If you encourage other Nato countries to participate, Putin is done for, and Russia will change. It’s the only answer.”
Andrew Foxall, Russia director at the Henry Jackson Society think tank, agreed, saying:
“While adopting financial scansions is important, it is completely right that the use of travel bans would hit Putin the hardest. The pressure on him to come to terms would be enormous.”
Andrew Foxall, Russia director at the Henry Jackson Society think- tank, said: “The use of travel bans would hit Putin the hardest. The pressure on him to come to terms would be enormous.”
Dr Felshtinsky said the Skripal attack was the culmination of a six- year operation, headed by Russia’s GRU military intelligence service.
He said: “His brother died in Moscow from an unknown illness; his son died in Moscow in 2017 through suspicious circumstances, his wife died in 2012 in Britain and his daughter was poisoned with him.
Putin’s lesson after the murder of Litvinenko is that there is no punishment, says Dr Felshtinsk
“Unlike the FSB, the GRU has no qualms about killing family members.
“The last time in Russia we saw this kind of killing was with the family of Leon Trotsky.”
Britain will consider financial sanctions against individuals and the Government is expected to back Labour calls for a money laundering bill lead- ing to so-called Magnitsky powers to punish human rights abuses with asset freezes and visa bans.
Sources said the expulsion of 23 Russian diplomats would leave Moscow with just seven spies at its London embassy. In 1971, Britain culled 105 agents in its biggest mass expulsion, which experts said showed the country was not a soft touch.
George Walden, the Foreign Office mandarin behind the action, who was later a minister under Margaret Thatcher, said last night there were many parallels between that Cold War operation and recent events. “On both occasions events were prompted by Russia’s rm belief that Britain was weak,” he said.
“Harold Wilson had embarked on detente with Russia and things had become rather lax.
“Of course, Russia took advantage, by bloating the number of diplomats they stationed at the embassy and delegation to more than 500, and as much again when wives were included. We suspected that only around half were there for diplomatic reasons.
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“Russia just didn’t take us seriously. We were hearing increasing reports about approaches – members of the Armed Forces would contact us to tell us they had been propositioned by Russians, who were clearly hoping to turn them.
“They were acting with a brazen carelessness which we still see in these modern times, such as the use of polonium in 2006 and nerve agents in Salisbury.”
But it was only after the defection of Russian spymaster Oleg Lyalin – and change of Government – the Walden could take action.
The “knitwear representative”, based at the Soviet delegation, was actually head of UK chief of Department Five, the KGB’s sabotage wing, responsible for drawing up contingency plans in the event of war with the West. Crucially, he knew the identity of every Russian agent in Britain.
“Lyalin gave us the names we needed and we acted,” said Walden.“The Russians were very surprised – they didn’t think we had it in us.”
Last night Dr Paul Maddrell, author of the new book Spy Chiefs, said; “Sadly things are not as simple now as they were then.
“Nowadays Russia has a full range of free market operations; the free market economy open to them. They can use anything, even organisations like RT.“
Anna Chapman – one of the 10 spies swapped for Sergei Skripal – is a classic example: she infiltrated Britain, married a UK citizen to change her name and then, armed with a British passport, went to the US which had been her real target all along.
Russia has expelled 23 British diplomats
“In 1971 Operation Foot was so successful that the KGB never recovered in Britain. I’m afraid last week’s expulsions will not have that effect.”
Reacting to the Russian reprisals Dr Andrew Foxall said: “Russia’s reaction shows that it is trying to close the for in the Salisbury affair and therefore its options are limited. The expulsion of 23 British diplomats is just tit-for-tat spycraft.
“The shutting of the British Council, however, shows again how usia responds by affecting its own people. In 2012, after the US adopted the Magnitsky Act, it banned Americans from adopting Russian orphans; in 2014, in response to sanctions, it scaled back the importing of cheap western food like Polish sausage, apples and milk. its the poorest in Russia who are hit hardest.”
Meanwhile, a poll found a third of Britons would back an England World Cup boycott.
The YouGov survey of 1,986 adults found 34 per cent of those questioned would support pulling Gareth South- gate’s team out of the nals in Russia this summer.
Thirty-nine per cent were against and 27 per cent unsure, while among football fans, 32 per centalso favoured a boycott and 56 per cent believed the team should take part. Twelve per cent said they were unsure.
The four-yearly extravaganza, due to kick off in Moscow on June 14, is expected to be used by Vladimir Putin as a massive PR opportunity for his country.
But Labour MP Stephen Kinnock said Britain should ask Fifa to post- pone the tournament to 2019 and take it away from Russia in retaliation for the Salisbury poisoning.