The two people wanted by the Novichok poisoning were members of Russia's "military intelligence service" acting under orders from the Russian state, British Prime Minister Theresa May said on Wednesday.
"Based on a body of intelligence, the government has concluded that the two people named by the police … are officers of the Russian military intelligence service, also known as GRU," he said.
"So this was not a dishonest operation, it's almost certain that it was also approved outside the GRU at a higher level of the Russian state."
Hours earlier, the Prosecution Service announced that Alexander Petrov and Ruslan Boshirov were indicted in absentia for conspiracy to commit murder, attempted murder and use of nerve agent Novichok against former spy Sergei Skripal and his daughter Yulia.
Prosecutor Sue Hemming said they had considered the evidence and concluded that "there is sufficient evidence to provide a realistic prospect of conviction and that it is clearly in the public interest to accuse Alexander Petrov and Ruslan Boshirov, who are Russian citizens."
Both men are around 40 years old.
However, he said that the United Kingdom did not ask Moscow to extradite the men because Russian law prohibits the extradition of the country's citizens.
Russia has denied any involvement in the March incident and suggested that the British security services carried out the attack to stoke the hysteria against Moscow.
The police detail the movements of the suspects
Neil Basu, head of the anti-terrorist police, said the two suspects were traveling under alias, but were around 40 years old and had genuine Russian passports.
Basu said there were traces of Novichok's contamination in the hotel room in London where the two men had stayed.
He said they arrived in Britain on March 2 and left on March 4.
They traveled by train to Salisbury two days in a row and believe that the first trip was a reconnaissance mission.
"The tests were carried out in the hotel room where the suspects had stayed.
Two swabs showed contamination of Novichok below levels that would cause concern for public health, "said Basu.
The couple left Britain on the night of March 4, the day the Skripals were poisoned.
Russia said on Wednesday it did not recognize the names of the two Russians named by the police as suspects.
"The names published by the media, like their photographs, mean nothing to us," Maria Zakharova, spokeswoman for the Russian Ministry of Foreign Affairs, told the TASS news agency.
The police have requested information about the two men, urging anyone who has seen them to contact the police.
The March poisoning sparked a diplomatic row that led to diplomatic expulsions from eye to eye between Britain and its allies and Russia, while the United States also imposed sanctions.
The Skripals and Bailey recovered, but on June 30, a British couple fell ill with the same type of nerve agent in the nearby town of Amesbury.
One of them, the mother of three children, Dawn Sturgess, 44, died on July 8.
His partner Charlie Rowley was discharged, but then returned to the hospital and is currently being treated for meningitis and vision loss.
The Organization for the Prohibition of Chemical Weapons (OPCW), a global watchdog, confirmed on Tuesday that Novichok was involved in its poisoning.
Police said on Wednesday that their case is now part of Skirpal's investigation.
"We do not believe that Dawn and Charlie were deliberately attacked, but that they became victims as a result of the imprudence with which such a toxic nerve agent was eliminated," Basu said.
Novichok is a military-grade nerve agent developed by the Soviet Union during the Cold War.