Prosecutor Sue Hemming said the UK is not asking Moscow to extradite the men because Russian law forbids extradition of the country's citizens.

British Prime Minister Theresa May pinned the blame on the Russian government, telling the House of Commons the men were members of the GRU, Russia's military intelligence agency.

"Just as the police investigation has enabled the CPS to bring charges against the two suspects, so the security and intelligence agencies have carried out their own investigations into the organisation behind this attack,'' she said.

"Based on this work, I can today tell the House that, based on a body of intelligence, the Government has concluded that the two individuals named by the police and CPS are officers from the Russian military intelligence service, also known as the GRU.

"The GRU is a highly disciplined organisation with a well-established chain of command.

"So this was not a rogue operation. It was almost certainly also approved outside the GRU at a senior level of the Russian state.''

Mrs May said 250 detectives had trawled through 11,000 hours of CCTV and taken more than 1400 statements to come to their conclusion.

"This was a sickening and despicable act in which a devastatingly toxic nerve agent - known as Novichok - was used to attack our country,'' she said.

"It left four people fighting for their lives and one innocent woman dead.''

Six months after Russian double-agent Mr Skripal and his daughter were poisoned with the military-grade nerve agent Novichok, police have produced CCTV of the men's movements around Salisbury and London.

They also released photographs of a fake perfume bottle they said was used to smuggle the deadly nerve agent into England.

While the Skripals were critically injured but ultimately survived the attack, two Salisbury locals who found the perfume bottle in June, Dawn Sturgess and Charlie Rowley, were also badly injured. Ms Sturgess died several days later.

Police released extraordinary CCTV images of the two men arriving off a Moscow into London's Gatwick Airport on March 2.

They also released images of them captured at Salisbury train station on March 3 in what they allege was a reconnaissance mission.

The men stayed at a hotel in London and travelled from Waterloo train station before being captured again on CCTV at Salisbury on March 4, the day the Skripals were attacked on March 4.

Finally, the police provided CCTV showing the men departing Heathrow Airport at 10.30pm on a Moscow-bound flight on March 4.

Police also produced photographs of a Nina Ricci perfume box, and a false perfume bottle inside with an extended nozzle, which they said had been brought over from Russia to smuggle the nerve agent into the UK.

Ms Sturgess and Mr Rowley were affected by the agent when Mr Rowley found it on the street and gave it to his partner Ms Sturgess, who sprayed it on her wrists.

The Sun reports that the 45-year-old Rowley is now totally blind and has lost the use of his limbs after being diagnosed with meningitis just weeks after he survived the attack.

Police also said the men's hotel room had returned dangerously-high levels of Novichok when police conducted tests for it.

The Crown Prosecution Service said the pair were also wanted for the attempted murder of a first-responder policeman, Nick Bailey, who was also badly injured by the nerve agent, which was sprayed from the perfume bottle onto the front door of Sergei Skripal's house in Salisbury.

Police also want to lay charges of using a chemical weapon contrary to the Chemical Weapons Act. But they say the men have never returned to the UK and they will not seek Russia's help extraditing them as Moscow has denied responsibility and made clear it will not extradite any of its citizens.

Scotland Yard Counter-Terror Commissioner Neil Basu said the charging of the two Russian nationals was "the most significant moment so far in what has been one of the most complex and intensive investigations we have undertaken in counterterrorism policing."