A Russian invasion "would not stop at Ukraine" and could lead to conflicts in other countries, Liz Truss has warned.
The Foreign Secretary urged Russian President Vladimir Putin to pull back from the brink as she issued a grim warning that military action could be "imminent".
Boris Johnson will chair an emergency COBR meeting today after he cut short a UK tour to deal with the Ukraine crisis.
The Prime Minister spoke to US President Joe Biden on Monday night, when both leaders agreed there was a "crucial window for diplomacy" but the threat of Russian aggression remained serious.
More than 100,000 Russian troops are massed on the Ukrainian borders. But some units started to return to their bases on Tuesday morning, offering a glimmer of hope for a diplomatic solution.
Ms Truss said Europe was teetering "on the brink of war" and time was running out to find a diplomatic solution.
And she said the crisis could embolden other countries with military ambitions.
The senior Tory told Sky News: "If we saw an invasion into Ukraine, there would be severe costs in terms of a long-running conflict, we could see the undermining of security more broadly in Europe and could see other aggressors around the world see it as an opportunity to expand their ambitions too.
"This is a very dangerous moment for the world."
Ms Truss said the stability of Europe was hanging in the balance.
"The big risk, of course, is if there is an invasion into Ukraine that will be hugely damaging for Russia and Ukraine. And it will further undermine the stability of Europe," she said.
"This, I fear, would not stop at Ukraine. This is an attack on the neighbouring states of Russia and other east European countries in trying to undermine legitimacy of them being part of NATO."
She said she was "very worried" about the situation, adding: "We could be on the brink of a war in Europe, which would have severe consequences not just for the people of Russia and Ukraine, but also for the broader security of Europe.
"There is still time for Vladimir Putin to step away from the brink. But there is only a limited amount of time for him to do that."
Mr Putin and his foreign minister Sergei Lavrov have agreed to continue down the path of negotiations, prompting hopes that there could still be a way to avert war.
A Downing Street source said there was "some hope" that diplomacy could work and Russia could step back.
The UK and its Western allies have warned that any Russian incursion into Ukraine would be met with an unprecedented package of sanctions
