Boris Johnson heads to Ukraine on Tuesday after missing a call with Vladimir Putin because he was in the Commons being grilled by MPs on Partygate.
Mr Johnson travels to Kyiv for talks with Ukraine’s President Volodymyr Zelensky as Putin’s forces continue their build-up.
No10 admitted a 4pm call with Putin had to be shelved.
Shadow Foreign Secretary David Lammy said: “A vital diplomatic opportunity has been missed as Boris Johnson scrambles to hold on to his job.
“These are the real world consequences of a distracted Prime Minister unfit for office.”
Mr Johnson said: “We urge Russia to step back and engage in dialogue.”
Foreign Secretary Liz Truss was due to go with the PM but last night revealed she had tested positive for Covid.
Speaking on Monday in Essex, Mr Johnson urged Moscow to de-escalate tensions.
"I think Russia needs to step back from the brink,” he said.
"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."
In a comment made before his trip, the PM said tonight: “It is the right of every Ukrainian to determine how they are governed.
“As a friend and a democratic partner, the UK will continue to uphold Ukraine’s sovereignty in the face of those who seek to destroy it.
“We urge Russia to step back and engage in dialogue to find a diplomatic resolution and avoid further bloodshed.”
Mr Johnson also announced £88million of aid to help Ukraine cut its reliance on Russian fuel supplies.
