Justin Welby, the Archbishop of Canterbury has said that "diplomacy should continue until the last moment in the tense Ukraine stand-off to allow both sides to pull back," singling out Russia's "completely unjustified threats."
Stressing the need for mediation, the Archbishop of Canterbury warned that once war flared all control would be "completely lost" and the "casualties will be terrible."
Speaking in Parliament, he also pressed the Government over provision being made to support fleeing refugees "should the worst come to the worst."
His comments came amid mounting fears of an invasion by Russia, which has amassed 100,000 troops along the border with its former Soviet neighbour.
Addressing the Tory Lords leader Baroness Evans of Bowes Park following a statement on the Ukraine crisis, Mr Welby said: "Would she agree that mediation and diplomacy should be pursued until the last moment, seeking to find ways of giving both sides the opportunity to withdraw, particularly the Russians to withdraw from their completely unjustified threats?
"Because the one thing we can be sure of is once a war starts all control of the situation is completely lost, possibly for years, and the casualties will be terrible."
He added: "What provision is the Government making to support, should the worst come to the worst, the very large numbers of refugees and the huge needs for humanitarian support that will inevitably be part of fighting in the late winter in eastern Europe?"
Responding, Lady Evans said: "Diplomacy is the only way out of the current situation, but Russia must uphold to the international agreements it has freely entered into and respect the sovereignty of Ukraine.
"We are and do remain open to efforts by Russia to reduce tensions and encourage it to engage with transparency and de-escalation mechanisms."
She highlighted the offer of US President Joe Biden to meet with his Russian counterpart Vladimir Putin, as part of attempts to de-escalate the situation.
"So there is a lot of effort going in," said Lady Evans, who also pointed to £40 million being provided by the UK to help Ukraine.
Former head of the Royal Navy Lord West of Spithead warned against the making of empty threats to the Kremlin and stressed the need for co-ordination among allies.
Coming against a backdrop of defence cutbacks, the Labour former security minister said: "Sabre rattling is not very good when the number of sabres you have got seems to be getting less and less all the time."
He added: "I believe that Putin has been boxed into a corner now. He doesn't want to get into a quagmire in the Ukraine.
"I believe it is incumbent on us to actually try and ease his escape from the corner that he is in by negotiating or renegotiating an agreement. And that I am afraid will mean you have to have concessions on both sides... but we mustn't concede of course any fundamental principle."
The Cardinal Vincent Nichols recently joined his brother bishops from the London area alongside the Papal Nuncio His Excellency Archbishop Claudio Gugerotti for a short service of prayer at the Ukrainian Cathedral to pray for peace in Ukraine.
Days earlier Pope Francis had called for a day of prayer on Wednesday January 26.
This resulted in a live-streamed 12-hour marathon of prayer throughout the world in every archeparchy and eparchy of the Ukrainian Greek Catholic Church.