There are "very strong indications" that the murder of former spy Alexander Litvinenko in London in 2006 was linked to the Russian state, the BBC quoted a senior security offical as saying late Monday.
Litvinenko, a fierce critic of the Kremlin under former Russian president Vladimir Putin, died after suffering radiation poisoning thought to have been ingested through a cup of tea.
Britain wants to charge Russian lawmaker Andrei Lugovoi, another former spy, over the death but Russia will not extradite him, one of the key issues contributing to cool relations between the two nations.
"We very strongly believe the Litvinenko case to have had some state involvement," the unnamed senior security official was quoted as saying.
The BBC said this was the first time a senior official had publicly made such a link.
The source added that a foiled apparent assassination bid last year against exiled Russian oligarch Boris Berezovsky, who left Russia after falling out with Putin, showed "willingness to consider operations against people in the West."
Such incidents have serious repercussions, the source added: "(It) messes up the relationship big time."
Security service MI5 says there are the same number of Russian intelligence officers in London now as during Soviet times and that they, along with Chinese agents, "present the greatest concern".
British Prime Minister Gordon Brown met Russian President Dmitry Medvedev at the G8 summit in Japan Monday and pledged to raise "all the outstanding issues".
Litvinenko's widow Marina welcomed Britain's "principled stand" over the extradition of Lugovoi in a statement released following the meeting.