Foreign Secretary William Hague has called for a "firm and united" response from the European Union after diplomatic crisis talks on Ukraine failed to achieve a breakthrough.
Mr Hague said EU foreign ministers should press ahead with sanctions against Moscow after the Russians made clear they would back a referendum by Crimea to break away from Ukraine and join Russia.
US secretary of state John Kerry and Russian foreign minister Sergey Lavrov emerged from more than five hours of negotiations in London yesterday admitting they had no "common vision" for a way forward.
A defiant Mr Lavrov even drew comparisons with Britain's 1982 conflict with Argentina, after Britain and the US denounced the referendum as illegitimate.
"Everybody understands that Crimea for Russia is something really important," Mr Lavrov said. "It means immeasurably more for Russia than the Falklands means for the United Kingdom."
Mr Hague last night described the failure of the talks as "deeply disappointing" and condemned Russia's decision to support Sunday's "illegal, unconstitutional and illegitimate" referendum - which is widely expected to deliver an overwhelming vote to break away from Ukraine.
"The United Kingdom is strongly of the view that there must be a firm and united response from the European Union at the meeting of the Foreign Affairs Council on Monday, and that the time has come for tougher restrictive measures to be adopted as previously agreed by EU nations," he said.
"The door remains open for diplomacy and dialogue and we urge the Russian government to take action to lower tensions and enter into direct talks with the Ukrainian government.
"But we are clear that the flagrant violation of the sovereignty and territorial integrity of Ukraine cannot go unchallenged, and we stand with the Ukrainian people in their right to choose their future free of intimidation and interference."