SkyNews TV channel, Censor.NET reports.
The Prime Minister insisted the UK is "toughening" up its action against Moscow - and announced fresh asset freezes against the two men responsible for the dissident's murder at a London hotel in 2006.
He said Russia will be handled "with clear eyes and a very cold heart", and confirmed existing measures of expelling Russian diplomats and refusing to co-operate with Moscow's intelligence agencies will continue.
Cameron added that the government is "writing again to the prosecuting authorities to see what more can be done".
His remarks follow the publication of Sir Robert Owen's 300-page report, which said Dmitri Kovtun and Andrei Lugovoi were probably acting under the direction of Russia's FSB intelligence service when they poisoned Mr Litvinenko with radioactive polonium.
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