The Wireless Telegraphy Act 2006, an Act of Parliament, was described by the Anderson Review as, outside of RIPA, “the key statute allowing for the interception of communications.”  Sections 48 and 49 grant broad powers for the interception of communications to “the Secretary of State, the Commissioners for [HMCR], or any other person designated for the purposes of this section by regulations made by the Secretary of State.”  The use of this power is limited to necessary and proportionate circumstances, and in relation to national security, the prevention of crime, public safety or health, economic well-being or tax collection.
The Anderson Review explains that “the relationship between the WTA and RIPA is somewhat opaque.” Since “there is no operational distinction between the two statutes … both [Acts] could be used to intercept the same communications.” 
 David Anderson Q.C., ‘A question of trust’, Report of the Investigatory Powers Review, July 2015, p.97.
 United Kingdom Parliament (2006), Wireless Telegraphy Act, Section 48(5), available at http://www.legislation.gov.uk/ukpga/2006/36/introduction.
 David Anderson Q.C., ‘A question of trust’, Report of the Investigatory Powers Review, July 2015, pp.97-98.