The details he passed on were so precise and so terrifying that intelligence agents had to drop some of their other investigations to concentrate on what was supposedly happening behind the net curtains of a neat terraced house in an East London suburb.

The belief was that the authorities had only days to act. Surveillance had to be hastily organised, the police and other agencies had to be told, along with ministers, that this time the terrorists were expected to use chemicals and not explosives to murder their victims.

The nervous informant claimed to have seen the chemical vest that the terrorist would use, and while he didn’t understand how the device would work, he did pass on a description and the address where he saw it. The man also offered a list of names.

Some elements of the story he had to tell agents bordered on the incredible, but security sources said that they dared not ignore this alert...