After President Putin signed a decree recognising Crimea as a sovereign state William Hague has said the referendum was not legitimate and its outcome was not legal.
The Foreign Secretary said:
We are witnessing a clear attempt to pave the way for the annexation of part of the sovereign territory of an independent European state, through military force and an illegal and illegitimate referendum.
The UK calls again on Russia to enter into dialogue with Ukraine and with the international community to resolve this crisis through diplomacy and in accordance with international law, not to exacerbate it further through unilateral and provocative actions.
Continuing to ignore those calls will bring serious consequences for Russia. We will urgently consider our response to this latest escalation with our allies and partners, including at the European Council this week.
Putin's decree recognising Crimea as a 'sovereign country' has triggered the toughest sanctions against Russia since the Cold War.
With over half of the votes counted, over 95% of people in Crimea have voted to become part of Russia, election officials have said.
At some point the West's warnings will have to become deeds, part of the problem is they don't know which Putin they're dealing with.