Boris Johnson says he will unveil a “first barrage” of economic sanctions against Russia later after it sent troops into eastern Ukraine.
The PM said the UK would “hit Russia very hard” after it sent soldiers into two rebel-held regions it recognized as independent states on Monday.
He added there was “a lot more we are going to do” if a full-scale invasion of the country takes place.
He is expected to set out more detail in the UK Parliament at 12:30 GMT.
Johnson said Russia had violated Ukraine’s sovereignty and “torn up” international law.
Russia has ordered troops into rebel-held Luhansk and Donetsk, two breakaway regions run by Moscow-backed separatists.
Russia has said the troops would be “peacekeeping” in the regions – a claim dismissed by the United States as “nonsense”.
The US and European Union are also considering sanctions against Russia, with EU ambassadors meeting to discuss possible punishments on Tuesday.
EU diplomats told the BBC proposals had been drawn up to target hundreds of Russians, including members of the Russian Parliament who voted to recognize Luhansk and Donetsk as independent.
A plan to sanction commanders of the Russian so-called “peacekeeping” mission has also been prepared, they added.
German Chancellor Olaf Scholz has already announced Germany will be suspending certification of the Nord Stream 2 gas pipeline to Russia.
Germany has been coming under increasing pressure to cancel the controversial project, which critics say is a tool of Russian foreign policy.
Fears over an invasion have been rising in recent months, as Russia has massed some 150,000 troops along Ukraine’s borders, according to US estimates.
Russia has been backing a bloody armed rebellion in the rebel-held regions for the last eight years, with fighting having claimed 14,000 lives.
Speaking after an early-morning emergency meeting of cabinet ministers on Tuesday, Johnson said “all the evidence” suggested Russian President Vladimir Putin was “bent on a full-scale invasion of Ukraine”.
He said the UK sanctions announced later would only be an initial step, as “we expect more Russian irrational behavior to come”.
He did not set out specific details but said sanctions would target bodies not just in Luhansk and Donetsk, but in Russia itself.
He added they would hit Russian economic interests supporting “Russia’s war machine” and its wider activities in the region.
He also said the UK would review the “lethal but defensive” military assistance it is providing to Ukraine.
Latvia’s foreign minister Edgars Rinkevics said sanctions would not stop a Russian invasion but could help stop “some kind of Soviet Union experience” in the long term.
The UK introduced a law earlier this month to widen the range of Russian businesses and people it can sanction – with MPs expected to sign off the changes later.
The government has also scrapped a visa scheme allowing rich investors fast-track residency, amid pressure to cut UK links to Russia.
What sanctions could be enforced?
- UK ministers have said sanctions could target companies and individuals linked to the Russian government
- Boris Johnson has also said the UK could limit Russian firms’ ability to raise capital on London’s markets
- He has also suggested Western allies were looking at stopping Russian companies trading in pounds and dollars
- Other measures Western nations could impose include restricting exports to Russia, or excluding it from the Swift financial messaging service