The UK government has promised to sanction Russia over President Vladimir Putin's decision to recognise two breakaway regions in eastern Ukraine.
Recent legislation will be used to impose restrictions on people and organisations linked to Russia, the BBC understands.
Government sources say sanctions would be "ratcheted up" in the event of a Russian incursion of Ukraine.
The PM is expected to set out further details in the UK Parliament later.
Health Secretary Sajid Javid accused Russia of a "flagrant violation" of Ukraine's sovereignty, adding it appeared an invasion of the country had already begun.
Speaking on BBC Breakfast, he said Europe was waking up to a "very dark morning", adding "we cannot allow this to stand".
He added UK sanctions would be targeted at individuals and companies linked to the Russian government, and Russia's economy.
Several of the UK's western allies, the US, France and the European Union, made similar pledges, condemning Mr Putin's move and promising sanctions.
Early on Tuesday morning, Prime Minister Boris Johnson discussed sanctions, as well as the latest intelligence, at a Cobra emergency response meeting of cabinet ministers and officials.
Mr Johnson told Ukrainian president Volodymyr Zelensky on Monday evening that he believed an invasion was a real possibility in the coming hours and days, a Downing Street spokesperson said.
In a phone call, the PM told Mr Zelensky he would "explore sending further defensive support to Ukraine" at the request of the country's government, as well as detailing sanctions.
A No 10 spokesperson added: "The leaders agreed that the West needed to support Ukraine in the event of an invasion, but should continue to pursue a diplomatic solution until the last possible second."
Hours earlier on Monday, Mr Putin signed a decree recognising the independence of the self-declared people's republics of Donetsk and Luhansk in eastern Ukraine.
Russian-backed rebels have been fighting Ukrainian forces in those regions since 2014 in a conflict that has claimed the lives of more than 14,000 people.
Mr Putin's decision has been preceded by weeks of tensions and intense diplomacy over a build-up of Russian troops on Ukraine's borders.
The decree signed by Mr Putin says Russian armed forces will perform "peacekeeping functions" in Donetsk and Lugansk.
In a lengthy TV address, Mr Putin, appearing visibly angry at points, claimed "modern Ukraine was entirely created by Russia" and was part of its ancient lands.
But in recent weeks, Russia has repeatedly denied it has plans to invade Ukraine and accused the West of "hysteria".
Mr Putin delivered the speech as UK Prime Minister Boris Johnson was giving one of his own in Downing Street on measures to lift Covid-19 restrictions.
Mr Johnson said Mr Putin's decision to recognise the two separatist Ukrainian regions was "plainly in breach of international law" and a "flagrant violation of the sovereignty and integrity" of the country.
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Mr Johnson said the decision was a "very ill omen" that things were moving in the wrong direction in Ukraine.
"The UK will continue to do everything we can to stand by the people of Ukraine with a very robust package of sanctions," he said.
He said it was becoming clear there was a need to "start applying as much pressure as we possibly can" because it was hard to see how the situation would improve.
Earlier this month, the UK government passed new legislation that expanded its powers to impose sanctions on those directly linked to Russian actions in Ukraine.
The legislation gave the government powers to impose sanctions on Russian businesses and individuals in strategically significant sectors, such as the chemical, defence, extractives, ICT and financial services industries.
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