British prosecutors prepared a request to extradite former KGB bodyguard Lugovoi Wednesday which Russia has rejected in advance.
Prime Minister Tony Blair's office urged respect for international law. Russia, however, said its law prohibiting the extradition of its nationals trumped any international agreement.
"An extradition request will be drawn up and it will be forwarded to the Russian government by our embassy in Moscow," a Foreign Office spokesman said on condition of anonymity in line with government policy.
Russia's ambassador in London was summoned to the Foreign Office on Tuesday, but the British spokesman refused to discuss any further contacts that may be under way. "Obviously, we have regular contact with the Russian Embassy in London anyway," he said.
Britain has received no formal response from Moscow, Blair's official spokesman said Wednesday, adding that a failure to produce Lugovoi risked worsening relations between the two nations.
The spokesman declined to say whether Blair or Foreign Secretary Margaret Beckett planned to speak directly to their Russian counterparts about the case.
Radioactive traces were found at a dozen sites across London after Litvinenko's death Nov. 23, including three hotels, the Arsenal team's soccer stadium, two airplanes and a business block used by self-exiled Russian tycoon Boris Berezovsky. More than 1,000 people in Britain and abroad were tested for contamination by polonium-210, the substance that poisoned Litvinenko.
On his deathbed, the 43-year-old Litvinenko said Russian President Vladimir Putin was behind his killing. Litvinenko had also said Russian authorities were behind a deadly 1999 apartment blast and the murder of investigative journalist Anna Politkovskaya.
The Russian government has denied all involvement in Litvinenko's death.
Britain has refused to hand 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.
It has also complained of growing numbers of Russian spies in Britain. Russia last year passed a law that allowed for security forces to use force abroad against people considered threats.
Britain's ambassador to Russia, Anthony Brenton, has complained of being stalked by a pro-Kremlin Russian youth group called Nashi, which wanted him to apologize for participating in an opposition conference last year.
Russia's Federal Security Service accused four British diplomats of spying after a report on state-run television said British diplomats had contacted Russian agents using communications equipment hidden in a fake rock in a Moscow park.
Energy is the most delicate issue.
Britain exports oil and gas, but depleting supplies have raised concerns about future reliance on the Gulf states and Russia.
The European Union 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.
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 and for Berezovsky.
"I consider this decision to be political, I did not kill Litvinenko," Lugovoi said.
Prosecutors said Lugovoi could be tried in Russia.