LONDON — Britain deepened its diplomatic feud with Moscow on Wednesday, charging two men it says are Russian military intelligence officers with the nerve-agent poisoning of Sergei Skripal, a double agent who betrayed the service by spying for the West.

But U.K. authorities acknowledged there was little chance Russia would hand over the suspects, Alexander Petrov and Ruslan Boshirov, to face justice in Britain.

Prime Minister Theresa May said the use of a chemical weapon in the city of Salisbury, which left a British woman dead and four people, including Skripal and his daughter, seriously ill, was carried out by officers of the GRU intelligence service and almost certainly approved "at a senior level of the Russian state."

"This was not a rogue operation," she told lawmakers after police released photos of the suspects as they traveled through London and Salisbury before flying back to Moscow from Heathrow Airport on the evening of March 4, hours after the Skripals were poisoned.

Moscow strongly denies involvement in the attack, and Russian officials said they did not recognize the suspects.

Russian Foreign Ministry spokeswoman Maria Zakharova said the names and images of Petrov and Boshirov "say nothing to us."

Russian nerve agent attacks
Packaging for a counterfeit bottle of perfume that was recovered from Charlie Rowley's home after he and his partner Dawn Sturgess were poisoned by the same nerve agent which was used to poison former Russian spy Sergei Skripal and his daughter Yulia in Salisbury, is seen in an image handed out by the Metropolitan Police in London, Britain September 5, 2018. Metroplitan Police handout via REUTERS

British prosecutors said the two were being charged in absentia with conspiracy to murder, attempted murder and use of the nerve agent Novichok.

Sue Hemming of the Crown Prosecution Service said the U.K. wouldn't ask Moscow to hand the men over because Russian law forbids extradition of its citizens. Britain has obtained domestic and European arrest warrants for the suspects, meaning they can be detained if they leave Russia for another European country.

Neil Basu, Britain's top police counterterrorism officer, conceded it was "very, very unlikely" police would be in a position to arrest them any time soon.

But, he said, "we will never give up."

Sergei Skripal, 67, is a former colonel in the GRU who was convicted in 2006 of spying for Britain and imprisoned. He was freed in a 2010 spy swap and settled in the U.K.

Skripal and his 33-year-old daughter Yulia were found unconscious on a bench in Salisbury, 90 miles (145 kilometers) southwest of London, on March 4. They spent weeks hospitalized in critical condition and are now recovering in a secret location for their own protection. A police officer, Nick Bailey, was also hospitalized.

British authorities and the international chemical weapons watchdog say the victims were exposed to Novichok, a type of military-grade nerve agent developed by the Soviet Union during the Cold War.

The poisoning ignited a diplomatic confrontation in which hundreds of envoys were expelled by both Russia and Western nations.

Six months after the chemical weapons attack rocked the quiet cathedral city, police released new details about what Basu called "one of the most complex investigations" the force had ever seen.

Police say Petrov and Boshirov, both about 40, flew from Moscow to London on Russian passports two days before the Skripals were poisoned. Basu said the passports were genuine but the names were probably aliases, and appealed to the public to help identify the men.

Police revealed that traces of Novichok were found at a hotel in London's east end where the men spent two nights.

Police didn't test the budget City Stay Hotel for Novichok until two months after the attack, but Basu said the tiny quantity of nerve agent found there did not pose a risk to other guests.

Police believe the nerve agent was smuggled to Britain in a counterfeit Nina Ricci perfume bottle and sprayed on the front door of Sergei Skripal's house.

More than three months later, the bottle was found by a local man, 48-year-old Charlie Rowley. He was hospitalized and his girlfriend Dawn Sturgess, 44, died after being exposed to the contents.

The Organization for the Prohibition of Chemical Weapons confirmed Tuesday that Rowley and Sturgess were also exposed to Novichok.

Police are still trying to determine where the bottle was between the Skripal poisoning in March and its discovery by Rowley on June 27. As a result, Basu said, police weren't yet ready to lay charges in the second poisoning, though the two Russians are the prime suspects.

Alexander Petrov and Ruslan Boshirov
Alexander Petrov and Ruslan Boshirov, who were formally accused of attempting to murder former Russian intelligence officer Sergei Skripal and his daughter Yulia in Salisbury, are seen in an image handed out by the Metropolitan Police in London, Britain September 5, 2018. Metroplitan Police handout via REUTERS

The case, with its chilling cloak-and-dagger details, echoes the 2006 murder of Alexander Litvinenko, a former Russian agent who died after drinking tea laced with radioactive polonium-210 at a London hotel. Britain spent years trying in vain to prosecute the prime suspects, Andrei Lugovoi and Dmitry Kovtun.

A British inquiry concluded that Litvinenko had been killed at the behest of the Russian state, probably with the knowledge of President Vladimir Putin.

Russian defense and security analyst Pavel Felgenhauer said authorization to attack the Skripals had also likely come "from the very top."

"This is a message to the Russian intelligence community and spy community that you do not sell out Putin to the West or there are going to be serious consequences," he said.

Western officials say Russia's intelligence services have grown increasingly aggressive in their overseas activities. Members of the GRU have been indicted in the U.S. for hacking the Democratic Party and Hillary Clinton's campaign during the 2016 U.S. presidential election.

May said Britain and its allies would "step up our collective efforts" against the agency, though she did not name any specific measures.

"There can be no place in any civilized international order for the kind of barbaric activity which we saw in Salisbury in March," she said.

"The Russian state needs to explain what happened in Salisbury," May added. "All we've had is obfuscation and lies."