Theresa May has launched her strongest attack on Russia yet, accusing Moscow of meddling in elections and carrying out cyber espionage.
Addressing leading business figures at a banquet in London, the prime minister said Vladimir Putin's government was trying to "undermine free societies".
Mrs May said it was "planting fake stories" to "sow discord in the West".
But, she added, whilst the UK did not want "perpetual confrontation" with Russia, it would protect its interests.
"Russia has repeatedly violated the national airspace of several European countries and mounted a sustained campaign of cyber espionage and disruption.
"This has included meddling in elections and hacking the Danish Ministry of Defense and the Bundestag among many others."
"We know what you are doing and you will not succeed. Because you underestimate the resilience of our democracies, the enduring attraction of free and open societies and the commitment of Western nations to the alliances that bind us."
She said as the UK left the EU and charted a new course in the world, it remained absolutely committed to Nato and securing a Brexit deal which "strengthens our liberal values", adding that a strong economic partnership between the UK and EU would be a bulwark against Russian agitation in Europe.