A Brexit trade deal would support Europe's commitment to open economies and free societies in the face of Russian threats to the international order, Theresa May has said.

The Prime Minister stressed a "comprehensive new economic partnership" with the European Union would "underpin" that "shared commitment" 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.

Addressing the Lord Mayor's Banquet at the City of London's Guildhall, Mrs May said: "As I said in my speech in Florence, 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."

Mrs May highlighted Russia's incursion in Ukraine, violations of the air space of several European countries and "a sustained campaign of cyber espionage and disruption", including "meddling in elections" and hacking foreign ministries and parliaments.

The PM said Russia is seeking to "weaponise information" by planting fake news and "photo-shopped images" in an attempt to "sow discord in the West and undermine our institutions".

She told Moscow: "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.