As reported by Censor.NET citing UNIAN, the GRU agent is believed to have visited Salisbury to help plan the attack before two of his colleagues brought military-grade nerve agent into the UK, according to the Telegraph.
It is understood that the man has now been identified by those investigating the planned hit in March, which inadvertently led to the death of local woman Dawn Sturgess.
It came as Jeremy Hunt, the Foreign Secretary, said he believed the Russians had made the attempt to kill the former spy because they felt they "got away" with the poisoning of Alexander Litvinenko in London in 2005.
"That's why Theresa May's reaction this time has been very different," Mr. Hunt told Sky News.
He said the Kremlin would not have expected 28 countries around the world to expel more than 150 Russian diplomats. His comments come after Ruslan Boshirov, one of the would-be assassins, was unmasked as highly decorated GRU colonel Anatoliy Vladimirovich Chepiga.
The true identity of Alexander Petrov remains unclear, though the Telegraph has established that he was using his real first name and an alias as a surname.
Now a third agent has been tracked in Salisbury and it is thought that he fed information back to Col Chepiga and Petrov. It comes after it was revealed by investigative journalist group Bellingcat that the numbers on the passports issued to Col Chepiga his accomplice in fake names were part of a series that have been linked to other GRU officers.
It is believed that the apparent blunder could have led to their international movements being traced and identities exposed. A reconnaissance mission would have been essential as an attack which saw nerve agent smeared on a door handle would not have been decided on a whim, security experts say.
The GRU officer is expected to have reported back on details such as the layout of the cul de sac and which door Mr. Skripal used to enter the property. Sergei Migdal, a security expert and former police and intelligence officer, said: "Someone would have had to carry out a reconnaissance mission beforehand.
"I understand that it was timed for Yulia's arrival as they wanted to be sure that their target would be in Salisbury. It would have been too much trouble to watch him all the time and that may have aroused suspicion. "We now know that there is a link between GRU officers and a series of passport numbers, and I would imagine that law enforcement has already looked at the border control records to establish where operatives have been."
Philip Ingram, a former intelligence and security officer, added: "There was almost certainly intelligence on the ground before the attack was carried out, it is what is called a pattern of life study. They wouldn't have turned up at the house and put it on the door handle on a whim."
Mr. Skripal and his daughter both survived the attack and are currently in hiding. The furore over the poisoning was reignited on Wednesday when Boshirov was unmasked as Col Chepiga, 39, who was made a Hero of the Russian Federation by decree of the president.
Mr. Ingram said: "This is a highly decorated officer who has clearly been fast tracked through his career and therefore it is right and proper that he would have been chosen to carry out such a mission, an international operation which is likely to have been personally authorised by Vladimir Putin.
"His boss, the head of the GRU Colonel General Igor Korobov, only got the Hero of the Russian Federation Award three years later in 2017, so it shows how highly decorated Col Chepiga was.
"Given his age he would have been fast tracked up the ranks, he was obviously seen as a rising star. "These two men were burned, their bosses knew that after this these individuals would never be able to operate overseas again.
"This colonel was obviously on the forefront for quite a number of years, Chechnya and Eastern Ukraine were really bloody, maybe he was a little bit burnt out and this was an excuse to give him one final mission before he goes off on a desk job."
The Metropolitan Police, which is leading the investigation, refused to comment.