A former Russian spy and his daughter were reportedly under surveillance by the Russian authorities months before they were poisoned in Salisbury.

Sergei Skripal and his daughter, Yulia, were exposed to the novichok nerve agent in March.

Britain accused Russia of poisoning the Skripals in the first known offensive use of such a chemical weapon on European soil since the Second World War.

Russia has denied any involvement in the poisoning.

Both the former spy and his daughter were under surveillance by the Russian authorities for some time before the attack, BBC Newsnight reports.

The broadcaster said Ms Skripal’s phone has been investigated for signs of malware which could have enabled it to be used to track her movements.

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

The British government previously said Russia had been spying on the Skripals for at least five years.

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

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

Mr Skripal may still have been seen as a target even after he was convicted of espionage and handed over to Britain in a high-profile spy swap, Mr Sedwill added.

Yulia Skripal's email account was targeted by Russia's intelligence agency, the British government said (PA)

It comes as counter-terrorism police said a couple left in a critical condition in Wiltshire were exposed to the same nerve agent.

Dawn Sturgess and Charlie Rowley were taken ill on Saturday in Amesbury, around eight miles away from where the Skripals were attacked with novichok in Salisbury.

Speaking to reporters from New Scotland Yard, the Metropolitan Police assistant commissioner, Neil Basu, said police were investigating links between the two poisonings.

“I appreciate that there will be a great deal of speculation as to whether this incident is linked to the events in Salisbury in March,” he said.

“I would add that the complex investigation into the attempted murders of Yulia and Sergei remains ongoing and detectives continue to sift through and assess all the available evidence and are following every possible lead to identify those responsible, for what remains a reckless and barbaric criminal act.

“I must say that we are not in a position to say whether the nerve agent was from the same batch that the Skripals were exposed to. The possibility that these two investigations might be linked is clearly a line of enquiry for us.”

Military personnel investigating the poisoning of Sergei and Yulia Skripal (PA)

Professor Dame Sally Davies, chief medical officer for England, said the Skripal episode meant officials had a “well-established response” in place.

She said: “I understand that those in Salisbury and in surrounding areas will be concerned at this news, particularly those who recently visited areas now cordoned off by police.”

She said the risk to the public remains low, but issued “highly precautionary” advice to those with concerns.

Personnel in hazmat suits investigate the poisoning of Sergei and Yulia Skripal (PA)

Around 100 counter-terrorism officers are working on the case and police have cordoned off at least five different areas, including a park and a property in Salisbury, as well as a pharmacy and a Baptist church community centre in Amesbury.

Mr Basu said: “This is a precautionary measure while we continue to investigate how they came into contact with the substance.”

The attack on the Skripals prompted the biggest Western expulsion of Russian diplomats since the Cold War, as British allies in Europe and the United States sided with the view of the prime minister, Theresa May, that Moscow was either responsible or had lost control of the nerve agent.