The Government is preparing further moves against Russia following a day of tit-for-tat action over the poisoning of former spy Sergei Skripal.
Speaking at the Conservative spring forum on Saturday, Theresa May said Britain "would never tolerate a threat to the life" of its citizens, or others in the country, from the Russian government.
"This act of Russian aggression is the very antithesis of the liberal and democratic values that define the United Kingdom," she said.
The pledge comes as 23 British diplomats based in Russia begin packing their bags as they are expelled from the country in retaliation for Russian representatives being told to leave the UK.
It is thought most or all are intelligence officers.
Russia has also shut down the offices of the British Council in Moscow – a move the cultural organisation says is "profoundly disappointing"
"It is our view that when political or diplomatic relations become difficult, cultural relations and educational opportunities are vital to maintain ongoing dialogue between people and institutions," a statement said.
We can now confirm the reports today that the British Council in Russia has been told to cease operations. We are profoundly disappointed at this development. Read more: https://t.co/xvMMEf6ywE
— British Council (@BritishCouncil) 17 March 2018
Russia has insisted that aggression is on the side of the UK, with the head of the country's foreign affairs committee calling the moves following the poisoning a "decision by the British side without evidence".
A tweet by the Russian Embassy in the UK said the 23 diplomats were expelled "to reach numerical parity" and that the British Council was closing "due to lack of legal grounds".
The escalating tensions come as Russia is preparing for its general election.
Other moves by the UK in response to the poisoning include legislative powers to defend against hostile state activity and the suspension of high-level contacts between the US and the Russian Federation.
Labour leader Jeremy Corbyn told the Labour conference that "Russian authorities must be held to account on the basis of the evidence" and called for Britain to "stop serving Russian crony capitalism".
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As the fallout from the poisoning unravels in the international arena, British police are continuing their investigation into the poisoning of Mr Skripal and his daughter Yulia.
They have renewed appeals for information about a burgundy BMW belonging to the former spy, which is believed to have been driven around Salisbury on the morning of the poisoning.