(CNN) UK Prime Minister Theresa May has mounted a stern attack on Russia, claiming that it interferes in elections and plants fake news stories to sow discord and undermine Western institutions.

In a significant escalation of rhetoric, May accused Moscow of attempting to "weaponize information" as part of a "sustained campaign of cyberespionage and disruption." Russia's actions were "threatening the international order," she said.

"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," May said in a speech at the annual Lord Mayor's Banquet in London on Monday.

Since its annexation of Crimea, May said Russia had fomented conflict in eastern Ukraine, violated airspace of European countries, and hacked the Danish ministry of defense and the German Parliament.

May said Britain would act to defend itself. "The UK will do what is necessary to protect ourselves, and work with our allies to do likewise," she said. "So we will take the necessary actions to counter Russian activity. But this is not where we want to be -- and not the relationship with Russia we want."

Read More