Speaking in Brussels ahead of a meeting of European Union foreign ministers tomorrow, Foreign Secretary William Hague condemned the vote as being in breach of the Ukrainian constitution.
"Nothing in the way that the referendum has been conducted should convince anyone that it is a legitimate exercise," he said in a Foreign Office statement.
"It is a mockery of proper democratic practice.
"The UK does not recognise the referendum or its outcome, in common with the majority of the international community," he added.
Crimea's pro-Moscow authorities announced that exit polls showed a 93 per cent vote in favour of becoming part of Russia.