The home secretary is expected to announce today that there will be a public inquiry into the death of Russian spy Alexander Litvinenko.
The ex-KGB officer was poisoned by a cup of tea laced with the deadly radioactive element polonium 210 during a meeting at a hotel in London in 2006 with two Russian agents. Litvinenko died in hospital three weeks later.
The investigation is set to examine whether the Russian state was behind his death.
Home Secretary Theresa May is due to publish a written letter to Parliament later today, overturning a previous government decision not to conduct an inquiry until results from a judge-led inquest were in.
But in 2013 a coroner ruled that a public inquiry would be better placed to find out how Litvinenko died.
His widow Marina has fought for a public inquiry since the incident, claiming that her husband had been working for MI6 at the time of his death. Litvinenko fled Russia in 2000 and was granted asylum in Britain.
Marina Litvinenko has said she is "relieved and delighted" that a public inquiry is to be held. "No matter how strong and powerful you are, truth will win out in the end," she said on Tuesday.
The Litvinenko affair caused significant diplomatic tensions between Britain and Russia. Today's announcement comes at a time of increasingly political fractures between the two countries as ructions intensify over Vladimir Putin's handling of the Ukraine MH17 disaster.