(Adds quote in para 2, 5, 6, background in para 3, more details in paras 7-8)

LONDON, July 16 (RIA Novosti) - Britain's foreign secretary said four Russian diplomats will be expelled from the country over Moscow's refusal to extradite a Russian suspect in the murder of former Russian security officer Alexander Litvinenko in London.

"We have chosen to expel four diplomats, four particular diplomats, in order to send a clear and proportionate signal to the Russian government about the seriousness of this case," David Miliband told parliament Monday.

Andrei Lugovoi, also a former security service officer, has dismissed British charges against him as political. He met Litvinenko on the day he fell ill in London.

Russian authorities said the response to British measures would be "appropriate", a source in Moscow said.

"If London resorts to such measures, Moscow will have the right to respond appropriately, basing its actions on international practice," the source said.

"As far as I know, Russia did not impose sanctions on the British diplomats caught for spying," he said, referring to a January 2006 incident with a group of British diplomats caught and filmed in Moscow with a spying electronic device.

The U.K.'s new foreign secretary also said the country would introduce visa restrictions for Russian officials, but not for tourists or other ordinary citizens.

He said trade with Russia, worth about $14 billion a year, would remain unaffected, but called on both sides to observe trade regulations.

Early last week, British prosecutors said they had received Russia's official refusal to extradite Lugovoi, which cited the Russian Constitution as saying Russian citizens could not be handed over to other countries, and proposed trying Lugovoi in Russia if Britain provides sufficient evidence.

But a spokesman for British Prime Minister Gordon Brown said the country's authorities could not be sure a Russian trial would be unbiased or sufficiently just.

Litvinenko died in a London hospital in November. British experts said they discovered radioactive isotope polonium-210 in his body, but published no official autopsy results. In his deathbed note, Litvinenko, who received British citizenship shortly before his death, blamed President Vladimir Putin for his murder, an allegation the Kremlin dismissed.