Britain made a bold extradition request for a former KGB bodyguard Tuesday in the poisoning death of an ex-Soviet spy turned Kremlin critic — a request Russia immediately refused — creating a stand-off with Europe's leading energy supplier and threatening to plunge relations to a post-Cold War low.

British prosecutors say they have enough evidence to charge Andrei Lugovoi with the murder of former KGB agent Alexander Litvinenko. Lugovoi met Litvinenko at a London hotel the day his tea was poisoned with polonium-210, a radioactive substance.

Lugovoi — a former KGB bodyguard — denied involvement Tuesday, saying the charges were politically motivated.

"Murder is murder; this is a very serious case," Prime Minister Tony 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."

The Russian ambassador was summoned to the Foreign Office on Tuesday. Blair's office said it expected full cooperation.

Radioactive traces were found at a dozen sites across London after Litvinenko's death Nov. 23, including three hotels, Arsenal's soccer stadium, two planes and a business block used by self-exiled Russian tycoon Boris Berezovsky.

In Britain, 700 people were tested for polonium contamination, while 670 were tested abroad. All were eventually released.

On his deathbed, the 43-year-old Litvinenko accused President Vladimir Putin of being behind his killing. He had also accused Russian authorities of being behind a deadly 1999 apartment blast and investigative journalist Anna Politkovskaya's murder.

The Russian government has denied all involvement in Litvinenko's death.

Although there is an extradition agreement between Russia and Britain, Russian law forbids the extradition of nationals.

A formal extradition request would be handed to the Russians later this week.

"In keeping with Article 61 of the Russian Constitution, a citizen of the Russian Federation cannot be extradited to another state," said Marina Gridneva, of the Russian Prosecutor General's office.

Lugovoi could be tried in Russia, prosecutors said. Litvinenko's widow, Marina, dismissed such a scenario.

"Everything that happened, happened here," she said.

Britain has refused to turn over Russian exiles, including Berezovsky — once an influential Kremlin insider who fell out with Putin and fled to Britain in 2000 to avoid a money-laundering investigation — and Chechen opposition leader Akhmed Zakayev.

Relations between Britain and Russia have long been sour.

British officials have complained of growing numbers of Russian spies operating within Britain. Last year, Russia passed a law that allowed for security forces to use force abroad against people considered threats.

Russia's Federal Security Service, meanwhile, 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 at a Moscow park.

"This is the latest wrinkle in our relations," said a British government official who spoke on condition of anonymity because of the sensitivity of the case. He said he doubted Britain would back down.

Britain's ambassador to Russia has recently complained of being harassed by a pro-Kremlin Russian youth group called Nashi.

Anthony Brenton has been heckled on speaking engagements and trailed by members of the group who follow him with banners, shout abuse at him and block his car.

Analysts say Britain's push for extradition could backfire.

Britain exports oil and gas but depleting supplies have raised concerns about future reliance on the Gulf states and Russia.

The EU gets a third of its oil and about 40 percent of its natural gas from Russia.

One fifth of the world's gas reserves are in Russia and are controlled by Gazprom, the giant Russian utility. Gazprom, which already has a minor presence in Britain, is targeting 20 percent of the domestic gas market by 2015.

"It's foolish," London-based ex-U.S. intelligence officer Bob Ayers said. "Russia is becoming a monopoly when it comes to energy supplies in Europe and the last thing you want to do is jeopardize that supply."

Russia's image abroad has suffered under Putin, who tried to build personal ties with leaders of Western countries, such as Jacques Chirac — many of whom are gone. Their predecessors, such as Angela Merkel, have taken a harder stance with Russia.

Even Blair has raised concerns about human rights abuses under Putin.

Stanislav Belkovsky, founder of the Moscow-based National Strategy Institute think tank, said the stand-off may further weaken Russia's image in the West but in the end, most countries will beware of angering Putin.

"If cornered, Putin could take unattractive and unpredictable actions with seriously dangerous consequences," Belkovsky said.

Lugovoi joined the KGB in 1987 after serving in the Kremlin guard corps. During his time in the KGB, he provided security for Prime Minister Yegor Gaidar, among others.

He also headed a group of guards for Berezovsky, who at that time was deputy head of the Russian Security Council, and later headed the security corps for ORT television, Russia's most widely broadcast channel. He now has a security company and has interests in the production of the Russian drink kvas.

"I consider this decision to be political, I did not kill Litvinenko," Lugovoi said. "I have no relation to his death and I can only express a well-founded distrust for the so-called basis of proof collected by British judicial officials."