Britain's Johnson plans to warn Putin as he bids to be leader of European solidarity

British Prime Minister Boris Johnson will warn President Vladimir Putin on Thursday against invading Ukraine, using a visit to the headquarters of the NATO military alliance to underscore what he cast as European solidarity against Russian hostility.

Russia, which has more than 100,000 troops at the Ukrainian border, denies it plans to invade. The United States, Britain, the European Union and NATO fear Putin may be planning to annex Ukraine.

As Johnson grapples with a domestic row over lockdown parties in Downing Street, he will on Thursday seek to cast himself as the European leader of attempts to avoid what could be the gravest security crisis on the Continent since at least the Cold War.

"The UK remains unwavering in our commitment to European security," Johnson, one of the leaders of the 2016 Brexit referendum campaign which led to Britain leaving the bloc.

"As an alliance we must draw lines in the snow and be clear there are principles upon which we will not compromise," Johnson said. "That includes the security of every NATO ally and the right of every European democracy to aspire to NATO membership."

While Johnson visits NATO and then Poland, Foreign Secretary Liz Truss will be in Moscow. Truss will visit Moscow State University and hold talks with Sergei Lavrov, the world's most experienced big power foreign minister.

Downing Street said Britain was leading international efforts to resolve the crisis, though French President Emmanuel Macron held hours of talks with Putin in the Kremlin on Monday.

Putin, who says that Moscow's concerns over NATO enlargement have been ignored for three decades, has demanded guarantees that there will be no missile deployments near its borders and no further enlargement of the military alliance.

The West says that NATO is a defence alliance, though Putin has repeatedly disputed this, citing wars in the Balkans, Iraq, Syria and Afghanistan.

Putin cautioned Macron at a news briefing in the Kremlin this week that if Ukraine ever joined NATO then there could be a war between Russia and the NATO alliance.

"If Ukraine will be in NATO and tries to return Crimea through military means then European countries will automatically be pulled into a military conflict with Russia," Putin told reporters.

Defence Secretary Ben Wallace told Times Radio that no one would win from the aggressive invasion of a sovereign state.

"What we're really all trying to do, whether you're in NATO or not in NATO, is protect the sovereign right of countries to choose their security alliances," Wallace said.