BORIS Johnson has admitted tough sanctions being imposed on Russian interests may not be able avoid an Ukraine invasion – as the Prime Minister warned Vladimir Putin may be “thinking illogically” about his imperial plans.

Mr Johnson has stressed that a Russian incursion across the border into Ukraine “could be the biggest war in Europe” since the Second World War, with casualties on both sides.

The PM, in comments made while at the Munich Security Conference on Saturday, said it appeared Moscow’s plan for an offensive had “already in some senses begun”, pointing to the escalation of violence in Russian-backed separatist-held areas in eastern Ukraine.

The UK has pledged support for Kyiv through armed forces training and by sending anti-tank weapons, but Mr Johnson said, given Ukraine is not a Nato member, sanctions would be the main focus in terms of hurting Russia in the event of an invasion.

He admitted that even a joint venture by Britain and the US to prevent Russian state-linked firms from trading in pounds and dollars – a move he predicted would hit the Kremlin “very hard” – may “not be enough on its own”.

Mr Johnson told the BBC’s Sunday Morning programme: “We have to accept at the moment that Vladimir Putin is possibly thinking illogically about this and doesn’t see the disaster ahead.”

Western fears about an invasion have become more pronounced in the past week, with US President Joe Biden stating on Friday he was “convinced” Mr Putin is preparing to order troops into Ukraine within days.

Mr Johnson appeared to back that conclusion, telling the BBC that was what the “evidence points to”.

He predicted a “bloody and protracted conflict” and called on his Russian counterpart to “engage in serious diplomatic conversation” to prevent a “sheer cost in human life”.

Concerns about an attack have been fuelled by rising violence in rebel-held areas of the country, with Ukraine’s military announcing that two soldiers died on Saturday as violence escalated in the east of the country between government forces and separatists.

There is anxiety that Russia, which has been carrying out military exercises across the weekend, including nuclear drills, could use the increase in tension in the separatist-held regions of Donetsk and Luhansk as a pretext for an attack.

The Prime Minister said: “The fact is that all the signs are that the plan has already in some senses begun.

“That’s what our American friends think and you’re seeing these provocations now in Donbas – these explosions and so on – that we’ve been warning about for a long time.

“The plan that we’re seeing is for something that could be the biggest war in Europe since 1945 just in terms of sheer scale.”