The Prime Minister pledged the UK will continue to play a leading diplomatic role after Brexit, suggesting a post EU divorce trade deal would support Europe's commitment to open economies and free societies in the face of Russian threats to the international order.
In a major foreign policy address at the Lord Mayor's Banquet, she said: "The UK will remain unconditionally committed to maintaining Europe's security.
"And the comprehensive new economic partnership we seek will underpin our shared commitment to open economies and free societies in the face of those who seek to undermine them.
"Chief among those today, of course, is Russia. 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."
Mrs May accused Russia of "seeking to weaponise information" as she criticised Moscow's meddling in elections and hacking of Denmark's defence ministry and the German parliament.
She also accused Moscow of "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".
"So I have a very simple message for Russia. We know what you are doing. And you will not succeed," she said.
"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."
The PM announced Foreign Secretary Boris Johnson will visit Moscow in the coming months for talks, as she promised Britain will "take the necessary actions to counter Russian activity".
But she added current tensions are "not the relationship with Russia we want".
"We do not want to return to the Cold War, or to be in a state of perpetual confrontation," she said.
"Many of us here looked at a post-Soviet Russia with hope.
"Because 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."
On the Middle East, the Prime Minister announced she would visit Jordan later this month to discuss its moves to deal with the challenge of refugees from Syria.
And claiming that Britain is at the forefront of international efforts in the fight against Islamic State and Islamist terrorism, Mrs May warned the Government would order military intervention where necessary.
"Of course we must never be paralysed by the myth that armed intervention is doomed to fail," she said.
"And the UK is not and will not be afraid to deploy its hard power where necessary."
Source : https://news.sky.com/story/theresa-may-launches-powerful-attack-on-russia-meddling-11125693