The father-daughter duo was in a critical condition for weeks after the poisoning.
Former Russian spy Sergei Skripal was allegedly poisoned by a nerve agent in Britain. Sixty-six-year-old Skripal and his daughter Yulia were found unconscious on a public bench in the southern English city of Salisbury on March 4.
British Prime Minister Theresa May said the Skripals were poisoned with Novichok, a deadly group of nerve agents developed by the Soviet military in the 1970s and 1980s, which was applied to his home's front door in liquid form. May blamed Russia for the poisoning.
It was the first known use of a military-grade nerve agent on European soil since World War II. Allies in Europe and the United States sided with May's view and ordered the biggest expulsion of Russian diplomats since the height of the Cold War.
Russia retaliated by expelling Western diplomats. Moscow denied any involvement and accused the British intelligence agencies of staging the attack to stoke anti-Russian hysteria.
Russian officials questioned why would Russia attack an ageing turncoat who was pardoned and swapped in a Kremlin-approved 2010 spy swap. President Vladimir Putin, himself a former KGB spy, said that Skripal would have been dead if he was attacked with a weapons grade agent.
The father-daughter duo was in a critical condition for weeks after the poisoning. Yulia, 33, was in a coma for 20 days.
Two Russian nationals were arrested after a few months in connection with the incident. A European arrest warrant was also issued for Alexander Petrov and Ruslan Boshirov.
Skripal, recruited by British spies while in Spain, ended up in Britain after a Cold War-style spy swap that brought 10 Russian spies captured in the United States back to Moscow in exchange for those accused by Moscow of spying for the West.