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
One of the greatest sporting events on earth gets underway this week as the Ryder Cup heads to Scotland, here's everything you need to know
A toddler was in need of an unusual rescue after she got a kettle stuck on her head while playing at home in Chile.
British Defence Secretary Michael Fallon is in the Middle East to gather support for the international military campaign against IS.