Russia has been spying on Sergei and Yulia Skripal for at least five years and investigated how to launch nerve agent attacks using door handles, Britain has said.

In a letter to Nato, national security adviser Sir Mark Sedwill revealed that they were the subject of “interest” from the Kremlin’s security services since 2013.

He said email accounts belonging to Ms Skripal had been targeted by cyber specialists with Russia’s GRU intelligence agency, where her father was a colonel while passing secrets to Britain as a double agent.

Sir Mark said that even after being convicted of espionage and handed over in a high-profile spy swap, Mr Skripal may still have been seen as a target.

“It is highly likely that the Russian intelligence services view at least some of its defectors as legitimate targets for assassination,” he added.

“We continue to judge that only Russia has the technical means, operational experience and motive for the attack on the Skripals, and that it is highly likely that the Russian state was responsible. There is no plausible alternative explanation.”

An independent investigation by the Organisation for the Prohibition of Chemical Weapons (OPCW) confirmed that a class of nerve agents known as novichoks were used in the assassination attempt.

Police said the substance was put on the front door of Mr Skripal’s home in Salisbury, poisoning him and his daughter on 4 March, along with one of the first police officers to respond.

Salisbury poisoning: Yulia Skripal discharged from hospital

Sir Mark said Russia investigated the delivery of nerve agents using door handles in a chemical weapons programme started in the 2000s, when special units were allegedly trained to apply the chemicals.

“Within the last decade, Russia has produced and stockpiled small quantities of novichoks under the same programme,” he added.

The OPCW said it had verified the UK’s analysis of the agent used but did not ascribe responsibility, sending the name and structure of the chemical to nations including Russia in a classified report.

It noted that the substance used was of “high purity” – a characteristic claimed by the British government to point to state involvement.

Sir Mark said the former Soviet Union developed novichok nerve agents primarily at the State Institute for Organic Chemistry and Technology at Shikhany, as part of a wider programme codenamed “Foliant”.

“It is highly likely that novichoks were developed to prevent detection by the West and to circumvent international chemical weapons controls,” he added. “The Russian state has previously produced novichoks, and would still be capable of doing so.”

Vladimir Putin’s government has repeatedly denied any involvement in the Salisbury attack, or breaking the Chemical Weapons Convention, and says all its stockpiles were destroyed.

But Sir Mark told Nato Secretary General Jens Stoltenberg that Russia’s chemical weapons programme continued after the collapse of the Soviet Union, and novichoks were being used by the military by the time it signed the convention in 1993.

Skripal attack aftermath – in pictures Show all 15 1 /15 Skripal attack aftermath – in pictures Skripal attack aftermath – in pictures Scene of attack Members of the emergency services in hazard suits fix the tent over the bench where Sergei and Yulia Skripal were found unconscious on a park bench in Salisbury in March 2018. Getty Skripal attack aftermath – in pictures Victim - Sergei Skripal The retired Russian colonel and former double agent for MI6 was in a critical condition in hospital for more than two months after being exposed to novichok in Salisbury. He was given refuge in the UK after being jailed in Moscow for treason. Mr Skripal came to Britain as part of a high-profile “spy swap” in 2010 in which four men were exchanged for ten Russian "sleeper agents" in the US. In this image he is speaking to his lawyer from behind bars in Moscow in 2006. AP Skripal attack aftermath – in pictures Victim - Yulia Skripal Yulia Skripal was struck down by a novichok poison alongside her father Sergei. Facebook Skripal attack aftermath – in pictures Scene of attack A police officer stands guard outside a branch of the Italian chain restaurant Zizzi where the pair dined at before falling ill. It was boarded off whilst investigators worked on the building and later found traces of the chemical weapon within it. AFP/Getty Skripal attack aftermath – in pictures Scene of attack Large areas of central Salisbury were cordoned off by police following the discovery of the Skripals. Traces of nerve agent were also found in The Mill pub. PA Skripal attack aftermath – in pictures Victim - Nick Bailey Detective Sergeant Nick Bailey, rushed to the aid of the Russian ex-spy and his daughter who were targeted with a nerve agent. He was hospitalized after aiding them and didn't leave until three weeks after the attack. Wiltshire Police/Rex Skripal attack aftermath – in pictures Police investigation - Skripal’s home Police believe they were poisoned at home, and detectives found the highest concentration of novichok on the front door of Mr Skripal’s house. Getty Skripal attack aftermath – in pictures Theresa May visits scene of attack Britain's Prime Minister Theresa May spokes with Wiltshire Police's Chief Constable Kier Pritchard near where the Skripal's were found. Britain expelled 23 Russian diplomats over the nerve agent poisoning and suspended high-level contacts, including for the World Cup on March 14. Theresa May told parliament that Russia had failed to respond to her demand for an explanation on how a Soviet-designed chemical, Novichok, was used in Salisbury. AFP/Getty Skripal attack aftermath – in pictures Skripal days before attack Sergei Skripal days before he was exposed to Novichok, that has left him fighting for life. ITV News Skripal attack aftermath – in pictures Police investigation - military involvement British soldiers were deployed soon after the attack to help a counter-terrorism investigation into the nerve agent attack. One of the places they were asked to help out with was Skripal's home and it's surrounding. They were asked to remove a vehicle connected to the agent attack in Salisbury, from a residential street in Gillingham. AFP/Getty Skripal attack aftermath – in pictures Police investigation Personnel in protective coveralls and breathing equipment cover an ambulance with a tarpaulin at the Salisbury District Hospital. AFP/Getty Skripal attack aftermath – in pictures Police investigation The investigation extended to the grave of Sergei Skripal's son Alexander in London Road cemetery. Getty Skripal attack aftermath – in pictures Police investigation The Counter Terrorism Policing Network requested assistance from the military to remove a number of vehicles and objects from Salisbury. EPA Skripal attack aftermath – in pictures Home Secretary visits scene of attack Home Secretary Amber Rudd visited the scene of the nerve agent attack at the Maltings shopping centre on 9 March. Getty Skripal attack aftermath – in pictures Yulia Skripal speaks for the first time Yulia Skripal, speaking for the first time, said she felt lucky to have survived the nerve agent attack in Salisbury which left her fighting for life. Ms Skripal said her life had been “turned upside down” by the assassination attempt. But the Russian national added she hoped to return to her homeland one day, despite the Kremlin being blamed for the attack. Reuters

He accused Russia of failing to report its work on novichoks at the time and continuing to develop the weapons after ratifying the convention, alleging that Vladimir Putin himself was involved in the programme in the mid-2000s.

“It is highly unlikely that any former Soviet republic other than Russia pursued an offensive chemical weapons programme after independence,” the letter concluded.

“It is unlikely that novichoks could be made and deployed by non-state actors such as a criminal or terrorist group, especially at the level of purity confirmed by OPCW.”

Citing the radioactive poisoning of Alexander Litvinenko, Sir Mark said Russia had a “proven record of conducting state-sponsored assassination”.

A public inquiry led by judge Sir Robert Owen, which concluded in 2016, said Russia had previously been involved in the assassination of its opponents using poisons.

The UK has called a meeting of the OPCW’s executive council on Wednesday, which will include several Nato members.

“I know that Nato will remain seized of the need to confront the increasingly aggressive pattern of Russia behaviour of which the attack in Salisbury was an acute and recent example,” Sir Mark said.

Ms Skripal, 33, is recovering in a secret location after being discharged from hospital, where her father remains seriously ill.

In a statement, she confirmed she had rejected a “kind” offer of consular assistance from the Russian embassy for the time being.

Tensions between Russia and the UK have worsened further over an alleged chemical attack in Syria (Syrian Civil Defence White Helmets via AP) (Syrian Civil Defense White Helmets via AP)

“I want to stress that no one speaks for me, or for my father, but ourselves,” Ms Skripal said.

“I am still suffering with the effects of the nerve agent used against us. I find myself in a totally different life than the ordinary one I left just over a month ago, and I am seeking to come to terms with my prospects, whilst also recovering from this attack on me.”

In a press conference at the Russian embassy in London, Ambassador Alexander Yakovenko expressed doubt over the authenticity of her statement.

“We didn’t produce novichok, we didn’t store this novichok, so-called under the Western classification, and it was never in our military forces,” he said.

“The investigation was conducted in the most non-transparent way ... we are not allowed to see our citizens, talk to doctors, have no idea about the treatment the Russian nationals receive.

“We cannot be sure that Yulia’s refusal to see us is genuine. We have every reason to see such actions as the abduction of two Russian nationals.”

The Russian embassy later published its own “report” into the attack, entitled “Salisbury: A Classified Case”, that repeated the denials and contained no significant new information.

Tensions between the UK and Russia have been heightened further by an alleged chemical attack by its Syrian allies on the rebel stronghold of Douma.