A second policeman had been poisoned in the attack on Salisbury Novichok, Scotland Yard announced today.
The Met said a Wiltshire policeman had been exposed to a small amount of the deadly nerve agent.
After "appropriate medical treatment", the unnamed official returned to work.
The official was involved in the reaction to the poisoning of Sergei and Julia Skripal in March last year.
The forensic examination of the official's blood sample was carried out immediately after taking up his duties. Only now can the troop confirm that traces of Novichok were found in the sample.
In a statement, Scotland Yard said, "These forensics tests were conducted as part of an ongoing investigation that was unprecedented in its complexity and size and comprised thousands of exhibits. The officers continue to work closely with scientists and experts to carefully and methodically review and investigate the evidence before us.
"The police officer has been informed and continues to receive support from the Wiltshire Police and other police and staff affected by the events in Salisbury and Amesbury last year."
The policeman is the fourth person to be considered ill after the first attack on Salisbury.
The scripters were seriously ill after the attack, as was Detective Sergeant Nick Bailey, who also fell ill.
A few months later, Dawn Sturgess and Charlie Rowley were also heavily exposed to Novichock in Amesbury. Mrs. Sturgess died a few days later.
The troupe added, "Detectives from the CTP network have also contacted a small number of other people whose blood samples were taken at that time to seek their consent to forensic analysis of the samples. There is no need for someone who has not been contacted by the police at this time to take action.
"The purpose of such a test is to support the criminal investigation and to confirm that the blood samples contain traces of Novichok. For this reason, further victims of the attack must be forensically identified.
"The forensic tests, however, have no health impact on these individuals or the general public."
Scotland Yard continues to investigate the poisoning. Two suspects, Alexander Petrov and Ruslan Boshirov, are still wanted by the police.
The two Russian men in their forties were charged with the nerve agent attack and a European arrest warrant was issued against the couple.