Boris Johnson said he will urge Vladimir Putin to “step back from the brink” when the pair hold crisis talks over Ukraine this week.
The Prime Minister said an invasion of Ukraine would be “an absolute disaster for the world”.
Mr Johnson is expected to visit the region with no sign of the tensions fuelled by the Russian military build-up on the border with Ukraine easing.
Speaking to reporters in Essex, Mr Johnson said: “What I will say to President Putin, as I have said before, is that I think we really all need to step back from the brink.
“I think Russia needs to step back from the brink.
“I think that an invasion of Ukraine, any incursion into Ukraine beyond the territory that Russia has already taken in 2014 would be an absolute disaster for the world, and above all it would be a disaster for Russia.”
Mr Johnson said that any Russian invasion would be “bitterly and bloodily resisted” by the Ukrainian people.
The Prime Minister’s comments came as part of a concerted diplomatic push to maintain the unity of Nato allies in the response to Russia.
Hungarian leader Viktor Orban, seen as one of Mr Putin’s closest partners in Nato and the European Union, is expected to travel to Moscow on Tuesday for talks which have led to concerns about the alliance being undermined.
Defence Secretary Ben Wallace, who was in Budapest for talks with his counterparts, insisted he supports Mr Orban’s visit, telling a press conference: “I think it’s very important that President Putin hears from a country such as Hungary that they will face direct economic consequences of any instability in the east.
“I don’t know what the prime minister of Hungary will actually say to Putin but I think it is clear that we all are in agreement that we don’t want instability, we don’t want war in the east, we don’t want casualties, we don’t want migrant flows, we don’t want high fuel prices and food prices which would inevitably follow from any actions.”
Foreign Secretary Liz Truss is expected to visit Ukraine this week and will fly to Moscow for talks within the next fortnight.
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She has promised new legislation announced to widen current sanctions so “there will be nowhere to hide for Putin’s oligarchs” if Russia invades Ukraine.
The UK is expected to bolster military presence in eastern Europe as part of Nato’s efforts to secure the region.
But British and Nato combat forces are not expected to be deployed in Ukraine, which is not a member of the alliance.
Around 100 British personnel are involved in a training mission in the country.
Mr Johnson said: “We have been there since 2014, training Ukrainian troops under Operation Orbital – about 21,000 we have trained.
“We give lethal but defensive weaponry to Ukraine.”
