"The steps taken by President Putin today to attempt to annex Crimea to Russia are in flagrant breach of international law and send a chilling message across the continent of Europe," Cameron said in a statement.
"It is completely unacceptable for Russia to use force to change borders, on the basis of a sham referendum held at the barrel of a Russian gun.
"President Putin should be in no doubt that Russia will face more serious consequences and I will push European leaders to agree further EU measures when we meet on Thursday," he said.