Mishkin had travelled to the United Kingdom under the name of Alexander Petrov.
Skripal and his daughter were found slumped on a public bench in the town Salisbury and a woman later died after her partner found a fake perfume bottle which police believe had been used to smuggle the nerve agent into the UK.
Bellingcat's findings also cast doubt on Russia's claims that Boshirov and Petrov were civilians and that the government had no knowledge of the Skripal attack.
The Skripal case and last week's revelations that the GRU also allegedly tried to hack into the world's chemical weapons watchdog in the Netherlands have further tattered Moscow's strained ties with the West.
They say it is the real name of Alexander Petrov, who the British government named as one of their key suspects, alongside Ruslan Boshirov, whose real name was revealed to be Anatoliy Chepiga.
The Bellingcat report says Mishkin, the doctor it said works for Russian intelligence, was born in 1979 in the Archangelsk District in northern Russia and graduated from the elite Military Medical Academies, where he was trained for medical work in the Russian navy.More news: Manchester City's Gabriel Jesus 'unhappy' at Riyad Mahrez taking penalty vs. Liverpool
Bellingcat says it "conclusively identified" Mishkin from "multiple open sources, testimony from people familiar with the person, as well as copies of personally identifying documents", including his passport.
Alexander Petrov and Ruslan Boshirov told RT's editor-in-chief they had nothing to do with the Skripals' poisoning.
Both suspects have appeared on Russian TV to deny any link to the Salisbury poisonings.
Footage released by London police of the two suspects in the poisoning of Sergei Skripal.
They explained that they went to the small town as tourists and had always wanted to see its tower.
The prosecutors said at the time the two were undercover GRU officers.
Putin on October 3 said that Skripal was a "scumbag" who had betrayed his country.
The British government has blamed Russian Federation for the attack.