The Big Brother Watch website has published a report into the scale of data losses across local authorities.
The anti-surveillance and pro-privacy organisation has documented more than 1,000 incidents across 132 local authorities, including at least 35 councils who have lost information about children and those in care.
It said "highly confidential information has been treated without the proper care and respect it deserves".
At least 244 laptops and portable computers were lost, while a minimum of 98 memory sticks and more than 93 mobile devices went missing.
But of the 1,035 incidents, local authorities reported that just 55 were reported to the Information Commissioner's Office. The report says only nine of the incidents led to staff or managers being sacked as a result.
Big Brother Watch said: "The growing volume of personal information held by local authorities is a significant threat to personal privacy and civil liberties. This report highlights how, despite data protection law, not enough is being done to ensure sensite information is held securely and protected."
Responding to the report, Grant Shapps, minister for local government, said:"I welcome this research by Big Brother Watch. This reinforces the need for steps to protect the privacy of law-abiding local residents.
"Civil liberties are under threat from the abuse of town hall surveillance powers, municipal nosy parkers rummaging through household bins and town hall officials losing sensitive personal data on children in care."
The councils exposed in the report for losing the most data were Buckinghamshire (72 incidents), Kent (72 incidents) and Essex (62). Northamptonshire and North Yorkshire were also in the top five.
One Buckinghamshire incident involved 2,000 email addresses in a mailshot being inadvertently being sent to the public. In Essex, childcare files were found in a hire car by a member of staff. In Kent, an outreach worker lost a memory stick containing school children's personal details and assessment results.
The incidents documented were a result of Freedom of Information requests made by Big Brother Watch to 434 local authorities between July 2008 and July 2011.