Alexander Litvinenko's widow said she is "relieved and delighted" that a public inquiry is to be held in her husband's 2006 death.
Referring to her husband as Sacha, the name her husband's loved ones knew him by, Marina Litvinenko said: "I am relieved and delighted with this decision. It sends a message to Sacha's murderers: no matter how strong and powerful you are, truth will win out in the end and you will be held accountable for your crimes.
She added: "It has taken nearly eight years to bring those culpable for Sacha's murder to justice. I look forward to the day when the truth behind my husband's murder is revealed for the whole world to see."
Last year, the government rejected a request for an inquiry into the killing of Litvinenko, who died after drinking tea poisoned with polonium - 210, a rare radioactive isotope.
In a letter to Sir Robert Owen, the current Coroner in the Inquest into Mr. Litvinenko’s death, May admitted "international relations" were a factor in the Government's decision.
Sir Robert will chair the new public inquiry.
More top news
Marks and Spencer will become the first UK retailer to add vitamin D to its entire packaged bread range.
An over-the-top 80's action movie parody featuring kung fu Hitler and a David Hasselhoff soundtrack gets almost four million views online.
The government is treating the fight against obesity as a priority, as a new report cautions that one in five cancer deaths is caused by it.