Police are believed to have identified the suspected perpetrators of the Novichok attack on a Russian ex-spy and his daughter in Salisbury in March, according to reports.
Several Russians were involved in the attempted murder of Sergei and Yulia Skripal, the Press Association says.
They are thought to have been identified through CCTV, cross-checked with border entry data.
Earlier this month, Dawn Sturgess, 44, died after being poisoned by Novichok.
She and her partner, Charlie Rowley, 45, fell ill on 30 June in Amesbury, Wiltshire. He remains seriously ill in hospital.
Police believe the incidents are linked. The UK government has blamed Russia, but the country's authorities deny any involvement.
Asked about the latest developments, the Russian ambassador to the UK Alexander Yakovenko told the BBC: "Unfortunately, we don't have official statement of the British side.
"I want to hear that from the Scotland Yard or from the Foreign Office. A lot of versions that we hear in newspapers, they are not supported by the Foreign Office."
Mr Skripal, 66, and his daughter, 33, who were discovered slumped on a bench in Salisbury on 4 March, have been discharged from hospital and moved to secure locations.
"Investigators believe they have identified the suspected perpetrators of the Novichok attack through CCTV and have cross-checked this with records of people who entered the country around that time," a source with knowledge of the investigation told the Press Association.
"They (the investigators) are sure they (the suspects) are Russian."
The Met Police, who are leading the investigation, have declined to comment. The BBC has not been able to independently confirm the story.
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