The government says it rejects the result of Crimea's referendum, which Foreign Secretary William Hague has denounced as a "mockery of proper democratic practice", bbc reports.
A total of 95.5% of voters in Crimea supported joining Russia and leaving Ukraine, officials said.
Mr Hague said Russia must now face "economic and political consequences".
A statement from Number 10 said that the UK did not "recognise" the referendum or its outcome.
Crowds of pro-Moscow voters celebrated in the main city of Simferopol, and Crimea's pro-Russia leader, Sergei Aksyonov, said he would apply to join Russia on Monday.
But some Crimeans loyal to Kiev boycotted the referendum, and the EU and US condemned it as illegal.
Mr Hague said: "Nothing in the way that the referendum has been conducted should convince anyone that it is a legitimate exercise.
"The referendum has taken place at 10 days' notice, without a proper campaign or public debate, with the political leaders of the country being unable to visit Crimea, and in the presence of many thousands of troops from a foreign country. 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."
With reports of more disturbances in the city of Donetsk in the largely pro-Russian eastern Ukraine, Mr Hague went on to warn the Kremlin against any further military incursion into Ukrainian territory.
"Any attempt by the Russian Federation to use the referendum as an excuse to annex the Crimea, or to take further action on Ukrainian territory, would be unacceptable," he said.
"I call 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."
'Resolve this crisis'
Mr Hague was speaking from Brussels where, on Monday, he is expected to discuss a range of possible sanctions with other EU ministers. These could include asset freezes, and travel bans aimed at senior Russian officials.
A Number 10 spokesman said: "We don't recognise the Crimea referendum or its outcome. We call on Russia to enter dialogue with Ukraine and resolve this crisis within international law."
Russia's military intervention in the Crimean peninsula - part of Russia until 1954 and host to its Black Sea fleet - followed the fall of Ukraine's pro-Moscow President Viktor Yanukovych on 22 February.
Even before the polling stations had closed, the referendum on its future was being widely denounced in the West.
In a joint statement, European Commission President Jose Manuel Barroso and European Council President Herman Van Rompuy said it was "illegal and illegitimate and its outcome will not be recognised".
There were 1.5 million eligible voters in the Crimean referendum, and election officials put the turnout in Sunday's vote at more than 80%.