LONDON — 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.

David Miliband told Parliament he had taken the steps because the Kremlin had failed to properly respond to the “horrifying and lingering” death of Alexander Litvinenko.

It was the first time since 1996 that Britain had used the sanction, which Russia vowed “will not go unanswered.”

“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,” Mr. Miliband told lawmakers at the House of Commons.

Moscow has refused to extradite a Russian businessman and former KGB agent, Andrei Lugovoi, to stand trial in London over the killing. Mr. Lugovoi has been named by British prosecutors as the chief suspect in the case.

Russia’s formal rejection was received a week ago by Britain’s Crown Prosecution Service, which in turn spurned a Russian offer to try Mr. Lugovoi in Russia.

“The heinous crime of murder does require justice,” Mr. Miliband said. “This response is proportional and it is clear at whom it is aimed.”

In Moscow, a spokesman for Russia’s Foreign Ministry, Mikhail Kamynin, said “the provocative actions conceived by the British authorities will not go unanswered and cannot fail to produce the most serious consequences for Russian-British relations as a whole.”

Mr. Kamynin said the expulsions were “a well-staged action to politicize the Litvinenko case” and claimed the British government was trying to justify its own refusal to extradite two prominent Kremlin opponents with asylum in Britain: a tycoon, Boris Berezovsky, and a Chechen separatist figure, Akhmed Zakayev.

Britain’s Foreign Office declined to specify the rank or position of the four Russian diplomats to be expelled, who had yet to leave the country.

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

A State Department spokesman, Sean McCormack, would not comment directly on the British decision, but he said America has urged cooperation between the countries.

“We believe that it is important to bring closure to that terrible crime,” Mr. McCormack told reporters. “We believe that it is important, as a matter of justice, to see some cooperation between the U.K. and Russia.”

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

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

A waiter who was working at the hotel said he believed a poison had been sprayed into a pot of green tea, according to a British newspaper report Sunday. Norberto Andrade told the Sunday Telegraph that when he later cleared the table, the tea looked more yellow than usual and became “thicker — it looked gooey.”

Mr. Miliband said Mr. Lugovoi had offered the tea to Litvinenko and that he later “suffered a horrifying and lingering death in front of his family. His murder put hundreds of others, residents and visitors, at risk of radiation contamination.”

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 Mr. Lugovoi. All were eventually released.

International agreements mean that Mr. Lugovoi could be extradited if he travels outside Russia, Mr. Miliband said.

Mr. Miliband said London has suspended visa facilitation negotiations with Russia and is reviewing cooperation on a range of issues. Britain and Moscow had been working on a process to speed up the issuing of visas, but will halt cooperation, the Foreign Office said.

Russian’s ambassador to London met with a senior aide to Mr. Miliband, Sir Peter Ricketts, shortly before lawmakers were told of the expulsions.

In March 1996, Moscow ordered out nine British diplomats, alleging that they were part of a spy ring. Britain expelled four Russians in response.