In his first news conference since he became Prime Minister, Gordon Brown said it is very important that Moscow understood the seriousness of the situation over the Litvinenko case.

"We want the Russian authorities to recognize, even at this stage, that it is their responsibility to extradite for trial the Russian citizen who has been identified by our prosecuting authorities," said Brown.

"We cannot tolerate a situation where all the evidence is that not only was one person assassinated, but many others were put at risk," Brown added.

Last Thursday, Russia expelled four British diplomats as response to Britain's expulsion of the same number to press its demand for Lugovoi's extradition.

Today Andrei Maiorov, deputy head of the department of extraordinary affairs in Russia's Prosecutor General's Office, defended Russia’s decision to refuse the extradition of Lugovoi.

"The British side rather selectively considers certain facts and information related to Litvinenko's murder," said Andrei Maiorov

"(Litvinenko), according to the (extradition) request, led an active battle against the work of Russia's Federal Security Service (FSB) and spoke critically of the current Russian authorities," he said.

Also the Russian prosecutors responded to British claims, saying that the British investigation is “flawed”

"I do not think our friends should criticize our justice system. I think their efforts would be better spent improving their own system," Alexander Zvyagintsev, Russia's deputy Prosecutor-General, told a news conference.

"The Russian side has more reasons to doubt the efficacy of the British legal system," Zvyagintsev added, saying that London's extradition request came after Moscow informed Britain of the constitutional ban.

The “diplomatic war” Russia and Britain has started after the British Foreign Secretary David Miliband announced last week the expulsion of four Russia diplomats.

The decision to expel the Russian diplomats was taken after Russia has refused to extradite murder suspect Andrei Lugovoi to face trial in Britain.

A former KGB agent, Andrei Lugovoi was named by the U.K. Crown Prosecution Service as the prime suspect in the murder of Kremlin critic Alexander Litvinenko.

Litvinenko, 43, died in London in November after being poisoned by radioactive polonium-210. UK requested Lugovoi extradition so he can stand the trial, but Russia refused to comply.

“A U.K. citizen has suffered a horrifying and lingering death,” Foreign Secretary David Miliband told Parliament in London last week.

Miliband said that according to the police Lugovoi had offered 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."

In response Moscow threw out four British diplomats.