Prime minister Boris Johnson told a news conference in Kyiv that the UK and allies are ready to launch sanctions against Russia "the moment the first Russian toe-cap crosses further into Ukrainian territory".
Boris Johnson said: "It is vital that Russia steps back and chooses a path of diplomacy.
"We are keen to engage in dialogue”.
Johnson also said the build-up of Russian troops on the Ukrainian border was possibly the greatest act of hostility "towards the Ukraine in our lifetimes".
He said any conflict would come at a high cost.
"The Ukrainian army will fight. They will put a very, very very fierce and bloody resistance and I think that parents, mothers in Russia should reflect on that fact," he said.
