British prosecutors requested the extradition Tuesday of Russian businessman Andrei Lugovoi to face a charge of murder in the poisoning death of former KGB agent Alexander Litvinenko, officials said Tuesday. Lugovoi met with Litvinenko at a London hotel only hours before the former agent turned Kremlin critic became ill with polonium-210 poisoning. Lugovoi has repeatedly denied any involvement in interviews with the police and media. The politically charged case has driven relations between London and Moscow to post-Cold War lows. Foreign Secretary Margaret Beckett summoned the Russian ambassador, and Prime Minister Tony Blair's spokesman said the government expected full cooperation. "Murder is murder; this is a very serious case," Blair's spokesman said, speaking on condition of anonymity in line with government policy. "The manner of the murder was also very serious because of the risks to public health." Blair's spokesman said Russia and Britain had a formal extradition agreement, but he declined to comment on previous claims from Moscow that it would not surrender its citizens to British authorities. The Kremlin declined to comment. Litvinenko, 43, died Nov. 23 after ingesting the rare radioactive isotope. On his deathbed, he accused President Vladimir Putin of being behind his killing. The Russian government denies involvement. The former agent had become a vocal Kremlin critic who accused Russian authorities of being behind deadly 1999 apartment building bombings that stoked support for a renewed offensive against separatists in Chechnya. Litvinenko also was a close associate of slain investigative journalist Anna Politkovskaya. "I would like to thank the police and the (prosecutors) for all their hard work in investigating the murder of my husband," Litvinenko's widow, Marina, said. "It is thanks to them that we have reached the point today of having a named person to be charged with this crime." The charge represents a new challenge to already tense relations between London and Moscow. In a speech last year to Russian ambassadors, Putin laid out his foreign policy goals and urged them to strengthen relations with the "leading" EU countries of Italy, France, Germany and Spain. Notably, Britain was snubbed. In January 2006, Russia's Federal Security Service, the FSB, accused four British diplomats of spying, after a state-run television report said British diplomats had contacted Russian agents using communications equipment hidden in a fake rock in a Moscow park. The FSB said one of the diplomats had provided money for non-governmental organizations and used the episode to justify a crackdown on NGOs. The Kremlin is also angry that Britain has given refuge to Boris Berezovsky, once an influential Kremlin insider under former President Boris Yeltsin, but who fell out with Putin and fled to Britain in 2000 to avoid a money-laundering investigation he says was politically motivated. Russian investigators questioned Berezovsky in a parallel investigation into the murder earlier this year. Berezovsky said that the charges against Lugovoi point directly to the Kremlin because such an audacious and complicated killing would not be possible without state support. "I am 100-percent sure that the British government understands the importance of this case," Berezovsky said.