It was intended to safeguard national security but has instead been used to spy on everyone from journalists to dog walkers.
The Regulation of Investigatory Powers Act (RIPA), introduced 15 years ago, defines the powers available to public bodies to carry out surveillance and investigation, and covers the interception of communications.
No one could argue that such powers are vital in the detection and prevention of terrorism but fears are growing that RIPA has been allowed to mushroom into what has been branded a snooper's charter.
When RIPA first became law just nine agencies were allowed to mount surveillance operations. However, the Act has regularly been "updated" and has now been extended to almost 800 bodies.
In 2010 Big Brother Watch claimed RIPA legislation was used an average of 11 times every day by councils in England, Wales and Scotland. Its report, entitled The Grim RIPA, attacked "absurd" use of the legislation.
In one case, Poole Borough Council spied on a couple after suspecting they had lied about their home address to get their daughter into an over-subscribed school. The couple, who were later exonerated, were shocked to learn the council had employed someone to watch their house and monitor their movements.
A separate survey also uncovered the widespread use of covert surveillance to combat littering, fly-tipping and dog-fouling.
More recently, concerns have been raised about RIPA being used to spy on journalists. Last October it emerged that police forces had used the Act to obtain information about reporters' sources in at least two cases.
The Metropolitan Police used RIPA to secretly obtain the phone records of The Sun newsdesk and its political editor Tom Newton Dunn to track down officers who leaked information to the paper about the Plebgate incident.
The paper had reported how the then Government chief whip Andrew Mitchell was alleged to have called police officers "f****** plebs".
Kent Police also used the legislation to obtain the records of the Mail on Sunday news editor and a reporter after the newspaper revealed how former Cabinet Minister Chris Huhne illegally conspired to have his speeding points put on to his wife's licence.
In November this newspaper reported how the PSNI was one of more than 40 UK police forces which refused to say if it had used RIPA to spy on journalists.