Canterbury Cathedral was evacuated after a man scattered white powder in one of its chapels.
Emergency workers wearing chemical protection suits were clearing up the powder and taking samples for analysis, Kent Police said. Paramedics were also on the scene. It was not immediately clear what the substance was.
Britain has been jittery about bioterrorism and other forms of attack since anthrax cases were confirmed in Florida, New York and Nevada.
"We are taking every precaution at this stage and so we have sent in firefighters with protective suits," said a fire department spokeswoman, speaking on traditional condition of anonymity. "It is still too early to say exactly what the powder is."
Canterbury Cathedral, in southeastern England, is the seat of the Archbishop of Canterbury, spiritual head of the Church of England.
Church staff began evacuating the cathedral at 2.30pm and police were searching for the man who allegedly dropped the powder.