Former Russian spy Alexander Litvinenko was "deliberately poisoned" and his murder was "probably" approved by Russian President Vladimir Putin, according to the report of a British public inquiry released Thursday.

Robert Owen, who led the investigation, said in the report that the operation to kill Litvinenko was "probably approved" by Putin.

"A number of the witnesses who gave evidence during the open sessions of the Inquiry expressed strong views as to President Putin's direct involvement in Mr Litvinenko's death," the report said.

Litvinenko died aged 43 at a London hotel in 2006, after he was reportedly poisoned with radioactive polonium. He fled to Britain in 2000 before he was granted asylum in 2001 and became a British citizen in October 2006. He had been a fierce critic of the Kremlin and Putin.

Russian foreign ministry spokesperson Maria Zakharova on Thursday refuted the allegations, saying that Britain's handling of the case was "opaque" and "politicized."

She added the process of the British inquiry was "not transparent, neither for the Russian side nor for the public", and the inquiry was from the beginning "trying to lead to a 'wanted' result."