Surveillance powers designed to tackle terrorism have been used by local councils more than 10,000 times - for "crimes" as minor as littering.
Details disclosed under the Freedom of Information Act show that councils in England and Wales used powers under the Regulation of Investigatory Powers Act (Ripa) to investigate offences ranging from dog fouling to taxi overcharging.
The Liberal Democrats, who obtained the details, said it represented a fresh erosion of civil liberties and warned that Ripa was becoming a "snooper's charter".
The findings are based on a survey of 182 district and unitary councils in England and Wales which responded to a freedom of information request.
They showed that Ripa powers have been used on 10,288 occasions since 2004, but just 9% of those inquiries led to a successful prosecution, caution, or fixed penalty notice.
The alleged offences included littering, illegal street trading, and taking the fairy lights from a Christmas tree.
Lib Dem local government spokesman Julia Goldsworthy said that when Ripa originally became law, only nine organisations - including the police and security services - were allowed to use it. That has since been extended to 795 bodies.
"This Government sees civil liberties as little more than a temporary inconvenience. Slowly but surely, freedoms have been eroded," she said.
"We're now in a situation where dog fouling is considered enough to warrant surveillance by council officials.
"Unless Ripa is reformed, it risks becoming a snoopers' charter. Surveillance powers should only be used to investigate serious crimes and must require a magistrate's warrant."
