South Lanarkshire Council posted the spreadsheet online in response to a Freedom of Information (FOI) request which asked for details of staff pay grades.
The spreadsheet was only supposed to include anonymised employee data, but details on a second page of the FOI response were not anonymised.
The employee data leak is understood to have included names, workplaces, salaries and national insurance numbers.
The data was published last month and only discovered last week.
