Skripal, 66, and his 33-year-old daughter Yulia, were found unconscious on a shopping centre bench in Salisbury in south-west England and are feared to have been poisoned with an "unknown substance", police said today.
They remain critically ill in hospital since they were discovered in the Wiltshire town on Sunday afternoon.
Russia has denied any information about what has been classified as a "major incident" by Wiltshire Police.
Police are looking for similarities between the cases and the modus operandi used, the Independent reported.
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The UK will respond "robustly" to any evidence of Russian involvement in the collapse of former spy Skripal, Foreign Secretary Boris Johnson warned today.
While it would be wrong to prejudge the investigation, I can reassure the House that should evidence emerge that implies state responsibility, then Her Majesty's Government will respond appropriately and robustly," Johnson said in the House of Commons in response to an urgent parliamentary question over the incident.
A "small number" of emergency services workers were also assessed immediately after the incident and one remains in hospital.
"The advice from Public Health England remains that, based on the evidence to date, currently there doesn't appear to be any immediate risk to public health," Wiltshire Police said, adding that it has increased police patrols in the city to reassure the public.
The latest incident sparked instant speculation of Russian-linked poisoning, given Sergei's past as a former double agent who passed Russian secrets to Britain's MI6 security services.
"We have to be alive to the fact of state threats," said Scotland Yard counter-terror chief Mark Rowley.
The former Russian colonel in military intelligence had been jailed in Russia for "high treason" in 2006 but was later given refuge in the UK as part of a spy swap deal in 2010 in exchange for UK-based Russian spy Anna Chapman and around 10 other so-called undercover "sleeper" agents.
British police efforts are now focussed on trying to find out what "unknown substance" harmed the pair, who were found slumped on a bench at The Maltings shopping centre in Salisbury by concerned passers-by.
A woman who saw Skripal and his daughter described the couple as appearing "out of it." Freya Church, a local resident, said she saw the pair on a bench.
"She sort of leant in on him, it looked like she'd passed out maybe," Church said.
"We don't possess any information about what could have been the cause [of Skripal's illness], and what this could be connected to," said Dmitry Peskov, a Kremlin spokesperson.
He said Russia had not been contacted by Britain but it was ready to assist in any investigation.
"Moscow is always open to cooperation," Peskov said, calling the incident "tragic".
Litvinenko's death had strained Britain's ties with Kremlin and his widow has described the latest incident as "deja vu" and called for those receiving political asylum to be "completely safe".
A public inquiry had found that her 43-year-old husband's killing had "probably" been carried out with the approval of Russian President Vladimir Putin, who had denied any Russian involvement and refused to extradite the two Russians accused by the British authorities of killing Litvinenko.
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