The nerve agent that poisoned a couple in Wiltshire could remain active for 50 years if left undisturbed in a sealed container, a counter-terror chief says.
Neil Basu told a meeting in Amesbury that police were working on the theory that Dawn Sturgess and Charlie Rowley had found a container housing Novichok.
Mr Basu said it was also “implausible” that the poisoning was not linked with the attack on the Skripals in March.
Ms Sturgess died on Sunday, while her partner Mr Rowley remains in hospital.
They both fell ill in Amesbury on 30 June after being exposed to Novichok.
Police are investigating a possible link to the case of Russians Sergei and Yulia Skripal – who were poisoned with the same substance in nearby Salisbury.
The UK government has blamed Russia for the incident, but the country’s authorities deny any involvement.
Chief Medical Officer Dame Sally Davies said people in the Amesbury and Salisbury area should not “pick up any foreign object which could contain liquid or gel, in the interests of their own safety”.
On Tuesday, Salisbury District Hospital said Mr Rowley, 45, had regained consciousness after a “small but significant improvement” and was in a critical but stable condition.
Mr Basu, the Met Police’s assistant commissioner for specialist operations, said scientific advice suggested that the nerve agent could remain active for decades.
“At this stage we cannot say with certainty that both the incident in March and this latest incident are linked,” Mr Basu said.
“Clearly that is our main line of inquiry, but our investigation must be led by the evidence available and the facts. I would need a forensic link to be definitive.
“This is a very rare substance banned by the international community and for there to be two separate, distinct incidents in one small English county is implausible to say the least.”
He said scientists at the Defence Science and Technology Laboratory at Porton Down will work hard to determine whether the nerve agents in the two incidents were from the same batch.
But, he warned, it may never be possible to establish a definitive link.
“I would love to be able to say that we have identified and caught the people responsible and how we are certain there are no traces of nerve agent left anywhere in Wiltshire,” Mr Basu said.
“But the brutal reality is that I cannot offer you any reassurance or guarantee at this time.”
Police are continuing to hunt for a contaminated container which they believe was handled by the pair.
The BBC understands Mr Rowley’s flat is regarded as the key location in the search, but Mr Basu added: “The brutal fact is we don’t know where they found it.
“I am hoping Charlie recovers, and when he recovers he will be able to tell us and perhaps shed some light on it which will narrow our search dramatically.”
Ms Sturgess, 44, lived in Salisbury, and the couple had been in the city before going to Mr Rowley’s flat in nearby Amesbury on Friday 29 June.
They both then fell ill on Saturday 30, and Ms Sturgess died eight days later. She had two sons, aged 19 and 23, and an 11-year-old daughter.
Public Health England has offered advice for people who may have visited one of five areas identified by police.
Those locations are Muggleton Road, Boots pharmacy, and the Baptist church in Amesbury, and John Baker House and Queen Elizabeth Gardens in Salisbury.
Anyone who was there between 22:00 BST on 29 June and 18:30 on 30 June should continue to follow advice, including washing their clothes in a washing machine.