LONDON, England (CNN) -- Britain is to expel four Russian diplomats in protest over Moscow's refusal to extradite a key suspect in the poisoning of a former KGB agent.

Andrei Lugovoi, above, is the main suspect in the murder of Russian emigre Alexander Litvinenko.

Foreign minister David Miliband said Monday Britain would review the level of its cooperation with Russia on a range of issues, including changing the way Russian government officials get visas.

"This is a situation the government has not sought and does not welcome. But we have no choice but to address it," Miliband said in a speech to parliament outlining the measures.

"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."

The Kremlin refuses to extradite Andrei Lugovoi, a former Russian state security agent, who British prosecutors wish to extradite to the UK over the suspected poisoning of Alexander Litvinenko.

Miliband said that international agreements meant Lugovoi could be extradited if he left Russia.

British prosecutors announced their decision to seek Lugovoi's extradition on May 22, but Russia formally refused to hand him over on July 9.

Relations between the two countries have deteriorated sharply since Litvinenko's death.

Moscow authorities have proposed putting Lugovoi on trial in Russia, but British prosecutors have rejected this, doubting Moscow's promises of a fair trial.

They allege Lugovoi used rare radioactive isotope polonium 210 to poison Litvinenko, a former Russian intelligence agent turned Kremlin critic, during a meeting in London in November.

Litvinenko became violently ill within hours of the meeting and died in hospital three weeks later on November 23.

Traces of polonium were found in locations frequented by Lugovoi before Litvinenko's death but he has insisted he is innocent, saying British intelligence or anti-Kremlin agents probably murdered Litvinenko.

Britain would also suspend attempts to speed up the visa process for Russian citizens, Miliband said.

"The government believes Russia is a key international partner for the UK," he said. "For all these reasons we need a relationship based on trust and mutual respect."

Litvinenko, a former employee of Russia's Federal Security Service fled to Britain in 2000 and became a fierce critic of President Vladimir Putin, accusing him of planning to kill his opponents, including journalist Anna Politkovskaya who was murdered in October 2006. E-mail to a friend

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