Russia's Foreign Ministry said on Thursday that Moscow does not consider the conclusions of a British inquiry into the 2006 death of a former Russian spy to be impartial because it claims the result had been predetermined, and said the inquiry had been "politicized".

"We regret that a purely criminal case was politicized and has darkened the general atmosphere of bilateral relations," Russian Foreign Ministry spokeswoman Maria Zakharova said in a statement.

A British judge pointed the finger at Russia for the radiation poisoning of former KGB agent Alexander Litvinenko.

The 300-page report said Russian President Vladimir Putin "probably approved" the killing. Britain on Thursday summoned the Russian ambassador after the report.

The inquiry's chairman, judge Robert Owen, said even before the start of the hearings last year that he believed there was evidence of "a prima facie case as to the culpability of the Russian state".

Litvinenko's widow Marina has led a long campaign for an inquiry and said it was "the last thing I can do for him".

"My struggle has been for the facts to be made public," she said, adding: "For me it is important to finally have an official explanation".

'Nuclear terrorism'

Litvinenko was allegedly poisoned at a hotel by a cup of tea laced with polonium-210 - an extremely expensive radioactive isotope only available in closed nuclear facilities - in a sequence of events which could have come from a Cold War thriller.

The ex-KGB agent turned freelance investigator and Kremlin critic publicly accused Putin of ordering his killing before he died in agony three weeks later on Nov 23, 2006.

The killing caused outrage after radioactive traces were found at various sites around London and it was dubbed by the media as the world's first act of "nuclear terrorism".

The original police investigation led British prosecutors to demand the extradition from Russia on murder charges of Andrei Lugovoi, a former Kremlin bodyguard who had tea with Litvinenko at the Millennium Hotel in London's upmarket Mayfair area.

Russia has refused to extradite him.

AP - AFP