A former Russian spy and his daughter were reportedly under surveillance by the Russian authorities months before they were poisoned in Salisbury.
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.
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.
It comes as counter-terrorism police said a couple left in a critical condition in Wiltshire were exposed to the same nerve agent.
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.”
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.
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.
Mystery surrounds the attack and the motive behind it remains unclear, as is the logic of using such an exotic nerve agent, which has overt links to the Soviet military during the Cold War.
Join our commenting forum
Join thought-provoking conversations, follow other Independent readers and see their replies