British Prime Minister Boris Johnson has called on Russia to hand over the Salisbury Novichok poisoning suspects after charges were authorised against a third spy.

Mr Johnson said the Russian government should recognise "our sense of justice must be done is not abated" after it was announced Denis Sergeev, who used the alias Sergey Fedotov while in the UK, faces a string of charges including trying to kill former Russian spy Sergei Skripal, his daughter Yulia and ex-police officer Nick Bailey.

Mr Skripal and his daughter Yulia were found unconscious, slumped on a public bench in Salisbury in March 2018, and they, along with a police officer who went to his house, were left critically ill in hospital from exposure to the military-grade nerve agent.

Britain charged two Russians with conspiracy to murder and attempted murder in September 2018, and said they had now charged another Russian with the same offences.

British police said all three were members of Russian's military intelligence service GRU, naming them as AlexanderMishkin, Anatoliy Chepiga, and Denis Sergeev.

Mr Sergeev, who used the alias Sergey Fedotov while in the UK, faces a string of charges including trying to kill the Skripals and former police officer Nick Bailey.

Mr Sergeev is accused of seven charges, including three of attempted murder as well as conspiracy to murder Mr Skripal, causing grievous bodily harm with intent to Ms Skripal and Mr Bailey, and possession and use of a chemical weapon.

These are the same counts faced by two other suspects in the case already identified by police in 2018 - Alexander Mishkin, who used the name Alexander Petrov while in the UK, and Anatoliy Chepiga, who used the alias Ruslan Boshirov.

Investigators say they now have evidence linking the three to Russian military intelligence service the GRU, and that the trio have been involved in similar operations in other countries including Bulgaria and the Czech Republic.

Alexander Petrov and Ruslan Boshirov

News website Bellingcat previously reported that Mr Sergeev is suspected of involvement in the poisoning of an arms manufacturer, his son and a factory manager in Bulgaria in 2015, while Mr Chepiga and Mr Mishkin are accused of being part of a squad behind an explosion at an arms depot in the Czech Republic the previous year.

Deputy Assistant Commissioner Dean Haydon, who is the senior national co-ordinator for UK counter-terrorism policing, said this morning: "The investigation team has been piecing together evidence that suggests that Petrov, Boshirov and Fedotov have all previously worked with each other and on behalf of the Russian state as part of operations carried out outside of Russia."

They had all visited the UK before 2018, although there is no evidence that this was for reconnaissance purposes, police said.

"All three of them are dangerous individuals," Mr Haydon said. "They have tried to murder people here in the UK, and they have also brought an extremely dangerous chemical weapon into the UK by means unknown.

"The amount of Novichok in that perfume bottle was quite significant and if it had come into the wider circulation of the public, without a doubt it would have killed hundreds if not thousands of people."

Mr Sergeev entered the UK at 11am on 2 March 2018, flying from Moscow to Heathrow and arriving about four hours before Mr Mishkin and Mr Chepiga landed at Gatwick.

The three met a number of times in the coming days, both out in the open and at indoor venues, but Mr Sergeev did not leave the capital.

No traces of Novichok were found in the hotel room that he stayed in for two nights, before flying back to Moscow from Heathrow on 4 March.

The poison was contained in a counterfeit perfume bottle, found in June that year in a charity shop bin in Amesbury by Charlie Rowley.

He was left seriously ill and his girlfriend Dawn Sturgess died after coming into contact with the deadly substance.

Dawn Sturgess died after coming into contact with Novichok

Police have appealed to the public for any information about how the bottle came to be found almost 13km from Salisbury, and where it was stored between the departure of the would-be assassins from the UK in March and its discovery three months later.

They believe that the three suspects identified so far are in Russia, and with no extradition treaty with the country there is little chance of them being brought to the UK.

Nick Price, CPS head of Special Crime and Counter Terrorism Division, said: "We will not be applying to Russia for the extradition of Sergey Fedotov as the Russian constitution does not permit extradition of its own nationals. Russia has made this clear following requests for extradition in other cases. Should this position change then an extradition request would be made."

Police will apply for an Interpol notice to be issued for Mr Sergeev aka Fedotov.

A number of other individuals remain under investigation over possible involvement in the Salisbury case.

No charges have yet been brought over Ms Sturgess' death, but Mr Haydon said the case remains a live investigation.

A pre-inquest review hearing into her death is due to take place tomorrow.

Earlier, the British prime minister told Sky News: "We do believe that all the suspects should be handed over for justice."

He said Russia has already paid a "heavy price" over the poisoning, with the expulsion of diplomats, adding:

"They should recognise that our sense of justice must be done is not abated.

"Dawn Sturgess, an innocent member of the British public, died in that event and we want to see those suspects handed over."

Speaking in the House of Commons, Home Secretary Priti Patel condemned the "appalling" attack, and said should the suspects ever leave Russia the UK would "take every possible step to detain and extradite them to face justice".

Additional reporting Reuters