The UK is charging two Russian men, identified as Alexander Petrov and Ruslan Boshirov, over the poisoning of a former Russian spy in March.

Prime Minister Theresa May said the two men were officers from Russia's intelligence services, also known as the GRU.

She said authorization for the attack "almost certainly" came from the senior levels of the Russian government.

The Russian government said the suspect's names meant "nothing to us."

Sergei Skripal, the former spy, and his daughter collapsed in Salisbury, England, after being exposed to Novichok, a military-grade nerve agent developed in the Soviet Union during the Cold War.

The police released a detailed description of the suspects' whereabouts in the run-up to the attack.

Britain is charging two Russian men over the poisoning of the former Russian spy Sergei Skripal in Salisbury, England, earlier this year.

Prosecutors said they had sufficient evidence to charge two men, identified as Alexander Petrov and Ruslan Boshirov, with attempted murder over the attack.

Prime Minister Theresa May on Wednesday added that the two men were officers from the Russian intelligence services, also known as the GRU.

"Security and intelligence agencies have carried out their own investigations," May told Parliament on Wednesday. "I can today tell the House ... that the government has concluded that the two individuals named are officers from the Russian intelligence services."

British police on Wednesday also released a detailed description of the suspects' whereabouts in the run-up to the attack as well as a series of images taken from surveillance footage of the two men in London and Salisbury.

Surveillance footage shows the two suspects leaving London for Moscow at Heathrow Airport hours after Skripal collapsed on March 4. London Metropolitan Police

Skripal previously worked as a military-intelligence colonel at the GRU but was recruited by British spies to pass on state secrets. He was later arrested and imprisoned but was pardoned and released to the UK by the Russian government in 2010.

May said authorization for the attack "almost certainly" came from the senior levels of the Russian government. She added that she would push for more European Union sanctions against Russia over the poisoning.

The two men are now believed to be in Russia. Authorities plan to formally request via Interpol that the Russian police arrest them.

Neil Basu, a senior officer with the London Metropolitan Police's counterterrorism unit, said that the two men most likely traveled under aliases and that Petrov and Boshirov might not be their real names.

Both suspects are estimated to be 40 years old.

Surveillance camera footage of Petrov and Boshirov in Salisbury, England, on the day the Skripals were poisoned. London Metropolitan Police

Russia: Suspects "mean nothing to us"

The Russian government has denied knowledge about the attack.

Presidential aide Yuri Ushakov told reporters on Wednesday, according to the state-run Tass news agency: "We heard or saw two names, but these names mean nothing to me personally."

"I do not understand why that was done and what signal the British side is sending. That is difficult to comprehend," he added.

Tass also cited Maria Zakharova, a spokeswoman for the foreign ministry, as saying: "The names and photographs published in the media say nothing to us."

Britain's diplomatic relationship with Russia suffered after London accused Moscow of being behind the Skripals' poisoning this March. The Kremlin has repeatedly denied knowing about the attack.

A composite photo of Sergei and Yulia Skripal. AP/Facebook

Skripal and his daughter, Yulia, collapsed in Salisbury in March after being exposed to Novichok, a military-grade nerve agent that was developed by the Soviet Union during the Cold War. The poison had been applied on Skripal's front door, police said.

Both father and daughter were eventually discharged from the hospital.

Poison in a perfume bottle

A British couple in Amesbury, a town near Salisbury, was exposed to the poison after coming into contact with a perfume bottle containing it in late June.

It resulted in the death of Dawn Sturgess, who fell ill after applying the substance to her wrists. The other victim, Charlie Rowley, was discharged from a hospital about two weeks after collapsing.

Rowley told the police he found a box he thought contained perfume in a charity bin in late June, more than three months after the Skripals collapsed.

The box contained a bottle, purported to be from the designer brand Nina Ricci, and an applicator, and Rowley got some of the poison on himself when he tried to put the two parts together at home.

A composite image showing a fake perfume box and bottle that contained the nerve agent intended to poison former spy Sergei Skripal. Photos of the two men suspected of carrying out the attack are inset. London Metropolitan Police; Business Insider

Tests run by the Ministry of Defense found that the bottle contained a "significant amount" of Novichok, the police said.

"The manner in which the bottle was modified leaves no doubt it was a cover for smuggling the weapon into the country, and for the delivery method for the attack against the Skripals' front door," May said.

The police on Tuesday said they thought the two incidents were linked.

Authorities said they believed the couple were not deliberately targeted but "became victims as a result of the recklessness in which such a toxic nerve agent was disposed of."

Surveillance camera footage of Petrov and Boshirov at a Salisbury train station the day before Skripal collapsed. London Metropolitan Police

The suspects' whereabouts

The police believe the two suspects were in the UK for just three days to carry out the attack. On Wednesday the force outlined the two suspects' whereabouts in the run-up to the Skripals' poisoning in March:

March 2, 3 p.m.: The suspects arrive at London's Gatwick Airport after flying from Moscow on Aeroflot Flight SU2588.

5 p.m. (approx): They travel by train into Victoria station, central London. They then travel on London public transport.

6 p.m. to 7 p.m.: They spend about an hour in Waterloo before going on to the City Stay Hotel in Bow Road, east London, where they stay for the next two nights.

March 3, 11:45 a.m.: They arrive at Waterloo station from their hotel, where they take a train to Salisbury, where Skripal lives.

2:25 p.m.: They arrive at Salisbury. The police believe this trip was for a reconnaissance of the area and do not believe they posed a risk to the public at this point.

4:10 p.m.: They leave Salisbury and arrive at their hotel four hours later.

March 4, 8:05 a.m.: The two men arrive at Waterloo station again to go to Salisbury.

4:45 p.m.: They return to London from Salisbury.

10:30 p.m.: They leave London for Moscow from Heathrow Airport on Aeroflot Flight SU2585.

Skripal and his daughter collapsed on a bench at a Salisbury shopping center at about 4:15 p.m. on March 4.