Alexander Litvinenko died of radiation poisoning in 2006 three weeks after ingesting polonium in a London hotel. His death triggered an international murder investigation implicating Russian president Vladimir Putin and naming two former KGB officers.
The murder is to be the subject of a public inquiry, announced by Theresa May earlier this week.
But in an intervention which has outraged the Litvinenko family, the spy's younger brother, Maxim, claims British agents were responsible.
His allegations are a dramatic U-turn from his original reaction, when he blamed Putin's Russia.
Last night, Litvinenko's widow Marina rejected them out of hand, accusing her brother-in-law of spreading lies that she said echoed the views of the Russian state.
In the days after his brother's death Maxim told the world he believed he had died by the orders of the Russian state, saying: 'My brother was one of Putin's most dangerous enemies.'
Comment: An understandable sentiment coming so soon after his death, but not proof that Putin or the Russian government had anything to do with it.
However, two years ago he was approached by the Russian general prosecutor's office who convinced him that Russia was not to blame.