Fifteen secretive orders are in force allowing British spy agencies to collect large volumes of communications data, it has been revealed.
The measures were issued by Home and Foreign Secretaries on behalf of MI5 and GCHQ using a little-known legal provision between 2001 and 2016.
Under Section 94 of the Telecommunications Act 1984, communications providers can be issued with directions which enable security and intelligence services to obtain communications data in bulk
Bulk powers cover a range of techniques used by GHCQ, MI5 and MI6 to acquire information in large volumes. They have been described by the Government as "essential" to counter-terrorism efforts.
The use of Section 94 directions to acquire bulk communications data - the who, when and where of a phone call or email but not the content - was only officially confirmed in public for the first time in November.
Now a glimpse into the regime has emerged after it was examined by a watchdog.
A report published on Thursday by the Interception of Communications Commissioner's Office (IOCCO) disclosed that there were a total of 23 "extant" section 94 directions within the scope of its oversight.
They were all given by the Home Secretary or Foreign Secretary at various times between 2001 and 2016 on behalf of MI5, GCHQ, the three intelligence agencies collectively - MI5, GCHQ, and MI6 - or the Metropolitan Police's counter-terrorism command.
Fifteen of the directions relate to the acquisition of bulk communications data, while the remaining eight directions relate to the provision of services in emergencies, for "civil contingency purposes" or to help agencies in safeguarding the security of their personnel and operations.
Only GCHQ and MI5 have section 94 directions to acquire bulk communications data, the report said.
In 2015, MI5 made 20,042 applications to access communications data obtained under section 94 directions.These applications related to 122,579 items of communications data.
Last year, GCHQ identified 141,251 communications addresses or identifiers of interest from communications data acquired in accordance with section 94 directions which directly contributed to an intelligence report.
Section 94 provides that any Secretary of State may give to a Public Electronic Communications Network (PECN) "such directions of a general character as appear to the Secretary of State to be necessary in the interests of national security or relations with the government of a country or territory outside the United Kingdom".
All of the directions covered by the IOCCO specified that they were necessary in the interests of national security.
Although there must have been consultation with providers prior to the direction being handed down, once it is given, they must comply.
None of the section 94 directions under the IOCCO's oversight have been laid in Parliament and remain subject to statutory secrecy provisions, the report said - adding that this "severely limits what we can say publicly about the nature of the directions and the conduct undertaken in pursuance of any direction".
Interception of Communications Commissioner Sir Stanley Burnton described the review as "extremely challenging", adding: "Our report highlights clearly the difficulties when statutes are operated in secret and without codified statutory procedures.
"We make extensive recommendations that the intelligence and law enforcement agencies must implement to clarify and bring consistency to their procedures, to remedy the lack of record-keeping requirements and to ensure that we can oversee properly how section 94 directions are given and used."
The IOCCO said it welcomed the fact that "some of the inadequacies of the practices" set out in the report and areas where it made recommendations will be rectified by sections of the Government's landmark Investigatory Powers Bill.