The inquiry into the murder of former KGB spy Alexander Litvinenko was "biased and opaque", the Russian foreign ministry has said.
State media quoted the ministry as saying that the Litvinenko case was politicised by UK officials.
The ministry also said London's handling of the investigation had clouded relations between Russia and the UK.
Foreign ministry spokeswoman Maria Zakharova said: "We regret that what was a purely criminal case was politicised and has clouded the general atmosphere of our bilateral ties.
"The process...was not transparent for the Russian side or for society because of the way materials were examined behind closed doors under the pretext that they were secret."
Some of the key details of the Alexander Litvinenko case broken down into numbers.
Sir Robert Owen's report into the death of Alexander Litvinenko identified a number of reasons why he could have been killed.
Sir Robert Owen's report into the death of Alexander Litvinenko in London in 2006 was published on Thursday. Here are the key findings.