British authorities say they have enough evidence to charge two Russian men over the nerve-agent poisoning of ex-spy Sergei Skripal and his daughter Yulia in the English city of Salisbury.

Key points: Britain issues warrants for the arrest of two men over nerve agent attack

Britain issues warrants for the arrest of two men over nerve agent attack PM Theresa May says the men are Russian intelligence (GRU) officers

PM Theresa May says the men are Russian intelligence (GRU) officers Russia says the photos of the men is not evidence of a crime

Sergei Skripal, a former colonel in Russia's GRU military intelligence service, and his 33-year-old daughter, Yulia, were found slumped unconscious on a bench in Salisbury in March after a liquid form of the Novichok type of nerve agent was applied to his home's front door.

Former spy Sergei Skripal and his daughter Yulia Skripal both survived the attack. ( AP: Misha Japaridze (L) Facebook: Yulia Skripal (R) )

The pair survived the poisoning, and on Wednesday Britain's Crown Prosecution Service (CPS) said it had obtained a European arrest warrant for two men, Alexander Petrov and Ruslan Boshirov, over the attack.

Police say the men, both about 40, flew from Moscow to London on Russian passports two days before the Skripals were poisoned on March 4.

CCTV footage captured the pair travelling on public transport to a hotel in East London before catching a train to Salisbury on March 3 where it is believed they scouted out the area.

The pair made the same trip to Salisbury on March 4, where they were seen on CCTV in the vicinity of Mr Skripal's home, where it is believed they sprayed his doorknob with Novichok from a small bottle of perfume.

They then caught a train to Heathrow Airport the same evening and returned to Moscow.

Police say it is unlikely the men will be arrested. ( Metropolitan Police via AP )

Assistant Police Commissioner Neil Basu said the men were probably using aliases.

He appealed to the public "to come forward and tell us who they are".

Intelligence points to suspects being GRU agents: May

British Prime Minister Theresa May told Parliament she had information indicating the two men were GRU agents.

"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," she said.

"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."

Ms May said the attacks were designed to send a message to former Russian spies living abroad, and she referenced the 2006 murder of former Russian spy Alexander Litvinenko, who died after being poisoned with polonium.

"The actions of the GRU are a threat to all our allies and to all our citizens," she said.

"Together with our allies we will deploy the full range of tools from across our National Security apparatus in order to counter the threat posed by the GRU."

Britain will present its evidence on the attacks at a UN Security Council meeting on Thursday, a spokesman for Prime Minister Theresa May said.

The spokesman also told reporters that British officials had stressed at a meeting with Russia's charge d'affaires in London that Britain wanted those responsible for the poisoning of a former Russian spy and his daughter brought to justice.

"We have called for a security council meeting to take place on Thursday so we can update the council on the progress of the Salisbury investigation," he said.

CCTV captured Ruslan Boshirov and Alexander Petrov at the Salisbury train station on March 3. ( Metropolitan Police via AP )

Extradition not being sought

The UK was not asking Moscow to extradite the men because Russian law forbids the extradition of its citizens, CPS Director of Legal Services Sue Hemming said in a statement on Wednesday.

"Russia has made this clear following requests for extradition in other cases," she said.

"Should this position change then an extradition request would be made.

"We have, however, obtained a European arrest warrant (EAW) which means that if either man travels to a country where an EAW is valid, they will be arrested and face extradition on these charges for which there is no statute of limitations."

Mr Basu conceded it was "very, very unlikely" police would be in a position to arrest them anytime soon.

British officials have blamed the Russian Government for the poisoning, a charge Moscow has denied.

Police believe the nerve agent used to poison the Skripals was smuggled to Britain in a counterfeit Nina Ricci perfume bottle and applied to the front door of Mr Skripal's house.

The perfume bottle with an adapted nozzle police believe was used to spray Novichok on the front door of Sergei Skripal's home. ( Supplied: Met Police )

More than three months later, the bottle was found in nearby Amesbury by a local man, Charlie Rowley.

He was hospitalised and his girlfriend Dawn Sturgess died after being exposed to the contents.

Police believe Ms Sturgess and Mr Rowley were not deliberately targeted, but were in fact victims of recklessness in how the nerve agent was disposed of after the attempted assassination of Mr Skirpal.

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, Mr Basu said, police were not yet ready to bring charges in the second poisoning, however Ms May told Parliament that the two attacks were now under one investigation.

Names, photos of suspects 'tell us nothing': Russia

Mr Basu would not say whether police believe the suspects worked for Russian security services but, he said, "this was a sophisticated attack across borders".

Sorry, this video has expired Yulia Skripal says she's lucky to be alive (Photo: Reuters/Dylan Martinez)

The Russian Foreign Ministry called the charges "informational manipulation" and said the naming and release of photos of the two suspects "tell us nothing".

Russia has called for the UK to establish "practical cooperation between law enforcers" following the indictments.

Russian officials have previously questioned why Russia would want to attack an ageing turncoat who was pardoned and swapped in a Kremlin-approved 2010 spy swap.

Russian President Vladimir Putin, himself a former KGB spy, said earlier last month Mr Skripal would have been dead if he was attacked with a weapons-grade agent.

Sorry, this video has expired 'I went into the bathroom and found her in the bath': Novichok victim says the poison took 15 minutes to take effect

ABC/AP