This archival study of the interception warrant offers a history of a secret governmental practice. Despite the lack of a coherent archival record of interception warrants, some traces remain. They reveal the changing organisational structure in which secret surveillance was operationalised. Moreover, recurrent memos and policy notes reveal the organisational steps taken to preserve official secrecy. Disclosure of material to courts or MPs was prohibited, even as new technologies and changes in administrative law led to the conclusion within the Home Office that the legality of interception was questionable, at best. Anticipating the eventual result in Malone v UK (1985) before the ECtHR, a working party convened by the Home Office sought to design legislation to create an administrative review system that would ensure no complaints were ever heard in public. The results of this study contextualise recent developments in the law in the aftermath of the Snowden disclosures.