Public authorities have accidentally released personal information at least once a fortnight since 2009, according to analysis by mySociety.
The freedom of information website said they have dealt with 154 occasions of a council or other public body accidentally leaking private data.
The site says these figures are ‘the tip of the iceberg’ and are calling on councils to improve their procedures for handling private data.
It said in response to an information request, authorities often provide an Excel spreadsheet but fail to remove private information from hidden tabs or pivot tables. The data is often about vulnerable people such as those on a child protection register or those receiving benefits.
A spokesman Local Government Association said: ‘Councils take the handling of confidential data extremely seriously and staff are given rigorous training. On the rare occasions breaches occur, robust investigations and reviews are immediately undertaken to ensure processes are tightened.’