Addressing the annual Lord Mayor's banquet in London last night, May was blunt and outspoken in her condemnation of Russian tactics of meddling in elections and cyber espionage.
"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 Defence and the Bundestag (Germany), among many others," said May.
"It is seeking to weaponise information. Deploying its state-run media organisations to plant fake stories and photo-shopped images in an attempt to sow discord in the West and undermine our institutions," she said.
The prime minister said she wanted to relay a "very simple message" for Russia, that the UK was aware of what it was doing and it will not succeed because European democracies will fight back.
"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.
"The UK will do what is necessary to protect ourselves, and work with our allies to do likewise," she warned.
May said that while her government was determined to take all necessary action to counter Russian activity, a return to the era of the Cold War was not desirable for either side.
"We do not want to return to the Cold War, or to be in a state of perpetual confrontation...Russia can, and I hope one day will, choose this different path," she said.
Her warning came in the lead up to UK foreign secretary Boris Johnson's scheduled visit to Moscow before the end of the year.
Russia has been accused of running so-called "troll factories" that disseminate fake news and provocative posts on social media.
It has also been linked with a damaging drive to influence the outcome of international elections, including in the US.
(This story has not been edited by Business Standard staff and is auto-generated from a syndicated feed.)