The interception of communications includes listening to the calls made on a telephone or opening and reading the contents of a target's letters or e-mails. This is only allowed under the authority of a warrant signed by the Secretary of State (usually the Home Secretary) where they are satisfied it is necessary and proportionate.
These warrants are issued under the terms of Part I of the Regulation of Investigatory Powers Act 2000 (RIPA). Interception operations are carried out in accordance with the Code of practice for the interception of communications.
MI5 can also request communications data under RIPA Part 1 (Chapter II). As with interception, MI5 must show that obtaining such communications data is necessary for one of the purposes set out in RIPA, that it is proportionate to what we are seeking to achieve and that we have considered the collateral intrusion that may be caused. The Investigatory Powers Commissioner's Office (IPCO) also oversees our arrangements for access to this type of data.