Britain will expel four Russian diplomats over the Kremlin's refusal to extradite the key suspect in the murder of a former KGB agent fatally poisoned in London, the foreign secretary said Monday.

Britain has also suspended visa facilitation negotiations with Russia and is reviewing cooperation on other issues, Foreign Secretary David Miliband said. Russia has refused to extradite a Russian businessman to stand trial in London over the killing of Alexander Litvinenko.

"The Russian government has failed to register either how seriously we treat this case or the seriousness of the issues involved, despite lobbying at the highest level and clear explanations of our need for a satisfactory response," Miliband said.

International agreements mean that Andrei Lugovoi, also a former KGB agent, could be extradited if he travels outside Russia, Miliband said.

Britain's Foreign Office declined to specify the rank or position of the four Russian diplomats to be expelled.

"We have chosen to expel four particular diplomats in order to send a clear and proportionate signal about the seriousness of this case," Miliband said.

British prosecutors have named Lugovoi as the chief suspect in the case, and authorities are seeking to try him for Litvinenko's murder.

Litvinenko died Nov. 23 in a London hospital after ingesting radioactive polonium-210. In a deathbed statement, he accused Russian President Vladimir Putin of being behind his killing.

The ex-security agent said he first felt ill after meeting Lugovoi and business partner Dmitry Kovtun at London's Millennium Hotel.

Traces of polonium-210 were found at around a dozen other sites in London, including three hotels, a stadium, two planes and an office building.

In Britain, 700 people were tested for polonium contamination and 670 were tested abroad -- including Lugovoi. All were eventually released.

Russia's formal rejection was received a week ago by British prosecutors, who in turn spurned an offer from Moscow to try Lugovoi in Russia.