Theresa May has accused Russia of threatening the international order by “meddling” in elections and spreading fake news in an attempt to “sow discord in the West”.
The Prime Minister told Russian president Vladimir Putin “we know what you are doing” and insisted he would not succeed.
Mrs May also stressed that a comprehensive Brexit trade deal with the European Union will help underpin a shared commitment to open economies and free societies as Moscow seeks to undermine Western values.
She reiterated Britain’s “unconditional” pledge to maintaining European security, including a proposed post-Brexit pact with the EU, while warning of the “scale and nature” of the threat from Russia.
Attacking Moscow at the Lord Mayor’s Banquet at the City of London’s Guildhall, Mrs May said: “Russia’s illegal annexation of Crimea was the first time since the Second World War that one sovereign nation has forcibly taken territory from another in Europe.
“Since then, Russia has fomented conflict in the Donbass, 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, among many others.
“It is seeking to weaponise information. Deploying its state-run media organisations to plant fake stories and Photoshopped images in an attempt to sow discord in the West and undermine our institutions.
“So I have a very simple message for Russia.
“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.
“The UK will do what is necessary to protect ourselves, and work with our allies to do likewise.”
Mrs May also stressed the need to improve relations with Russia to avoid a return to the Cold War, saying: “While we must beware, we also want to engage.”
She evoked the hope that greeted the fall of the Soviet Union, saying Britain and Russia should not be “in a state of perpetual confrontation”, and announced that Foreign Secretary Boris Johnson will visit Moscow in the coming months for talks.
“We know that a strong and prosperous Russia which plays by the rules would be in the interests of the United Kingdom, Europe and the world.
“As a permanent member of the UN Security Council, Russia has the reach and the responsibility to play a vital role in promoting international stability.
“Russia can, and I hope one day will, choose this different path.
“But for as long as Russia does not, we will act together to protect our interests and the international order on which they depend.”