Russian agents responsible for the Novichok poisonings in Salisbury, England, sent a coded message to Moscow which included the phrase "The package has been delivered", it was claimed yesterday.
A British intelligence listening station based in Cyprus allegedly picked up the message shortly after former Russian double agent Sergei Skripal and his daughter, Yulia, were attacked in March.
The coded dispatch helped cement suspicions that it was the Russian state behind the attempted assassination, before counterterror police using facial recognition technology and CCTV images were able to identify those responsible.
It is believed the perpetrators flew back to Russia shortly after carrying out the hit, travelling to London from Salisbury by train.
The Daily Telegraph reported in April that key suspects had been identified following the attack.
Police are now exploring the theory that the assassins discarded a glass bottle containing traces of Novichok in Queen Elizabeth Gardens in the centre of Salisbury, which was discovered three months later by local couple Dawn Sturgess and Charlie Rowley.
The investigation became a murder inquiry this month when 44-year-old mother-of-three Sturgess died after she was accidentally poisoned by Novichok. Rowley, 45, is making a slow recovery and has been able to help police piece together the likely movements of the assassins before they fled Salisbury.
Sources close to the investigation have insisted the British authorities now have the names of those behind the poisonings.
But the Prime Minister's deputy official spokesman would not confirm whether the information had been presented to the Russians.
She said: "This is an ongoing complex investigation and I can't comment on the speculation around it. We need to make sure the police have the time and space to carry on with the investigation."
Maria Zakharova, the Russian Foreign Ministry spokeswoman, said Moscow has not been officially informed of the identities of the alleged suspects.
She said: "No contacts were initiated from the side of British law enforcement or official or diplomatic representatives about this topic today in principle.
"All the information we have is taken from media through leaks or statements. Our London embassy writes requests to confirm or deny this information but doesn't receive any answers."
An inquest into Sturgess' death was opened by the Wiltshire and Swindon Coroner on Thursday and was adjourned until January. Her body has been released for a funeral.
"The cause of death will not be given until further tests are completed," coroner David Ridley said.