London Hundreds of anti-terrorist police - some in biochemical suits - raided a house in east London on Friday and arrested two men, shooting and wounding one of them.
The raid was one of the largest in the capital since bombings of London's transport network in July killed 52 people. Police said they do not believe the suspected new threat was linked to those attacks.
Police did not release full details of the operation, but British media reported that authorities suspected that a chemical weapon had been built at the house for use against a British target.
Police did say that officers had not discovered any weapon or device.
Some of the roughly 300 officers in the raid wore protective chemical, biological and radiological suits.
A 23-year-old suspect was shot in the shoulder and was undergoing hospital treatment. It was unclear if he had been armed.