At 8.01pm on Wednesday, 1 December an officer on duty at the Palace of Westminster was sprayed with an unknown substance by a passing pedestrian who then made off in the direction of Victoria Tower Gardens.
A full search of the area was carried out with support from the police helicopter but at this time the person responsible has not been located.
The officer sustained mild irritation to his face and was assessed by the London Ambulance Service. He did not require hospital treatment.
Anyone who witnessed any suspicious activity in the area, or who has information about the incident, should call 101 with the reference 6588/01DEC.
Information can also be provided to Crimestoppers, anonymously, on 0800 555 111.