Russia will be “punished” if it launches an offensive on Ukraine, Downing Street has warned.
It comes as intelligence confirms that Moscow has amassed an estimated 100,000 troops on the Ukrainian border.
Last-ditch talks are being held between US Secretary of State Antony Blinken and Russian diplomats in Switzerland on Friday.
Number 10 said if Russian President Vladimir Putin launches an offensive, there will be a “package of sweeping measures” launched by the UK and its allies against the Kremlin.
A spokesman for Boris Johnson said: “I think the Prime Minister has been clear that any destabilising action by Russia in Ukraine would be a strategic mistake and would have significant consequences.
“I’ve talked about the fact that we’re working closely with our partners, including the US, to draw up a package of sweeping measures to make sure that the Russian government is punished if it crosses the line.”
Earlier, Foreign Secretary Liz Truss warned Mr Putin he must “desist and step back” from war in Ukraine or risk being dragged into a prolonged conflict like the Soviet invasion of Afghanistan.
She said: "The Kremlin hasn’t learned the lessons of history. They dream of recreating the Soviet Union, or a kind of ‘Greater Russia’ carving up territory based on ethnicity, and language. They claim they want stability, while they work to threaten and destabilise others.
"We know what lies down that path, and the terrible toll in lives lost and human suffering it brings. That’s why we urge President Putin to desist and step back from Ukraine before he makes a massive strategic mistake.
"Invasion will only lead to a quagmire, as we know from the Soviet-Afghan war or the conflict in Chechnya."
Ms Truss said as well as dealing with the immediate threat, the “free world” also needs to dial down its “economic dependence” on Russia.
She added: "Last week, at the NATO-Russia Council we sent a clear message to Moscow that any further incursion into Ukraine would bring massive consequences, including through coordinated sanctions hitting the financial sector, and individuals.
"This week, the United Kingdom announced a new package of training, support and defensive weapons for Ukraine to boost their defensive capabilities. And we’re working with our partners on high impact measures targeting the Russian financial sector, and individuals.
"We’re also strengthening our bilateral partnership following high-level talks in London in December – and we’re fostering new trilateral ties with Poland and Ukraine.
"We’re also pushing for alternatives in energy supply, so that nations are less reliant on Russia for their gas.
"We need everyone to step up. Together with our allies, we will continue to stand with Ukraine and urge Russia to de-escalate."
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