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.