Personal data of more than 6,500 people was leaked online after a council blunder.
Chelmsford City Council said it has informed those affected by the breach, which took place between 14 November and 14 December.
The council has apologised for the error which disclosed names, addresses and some national insurance numbers.
The Information Commissioner's Office (ICO) said it would not take any action against the authority.
The council revealed databases full of information provided by 6,771 customers through online forms had been uploaded and became publicly accessible.
But, it said, no bank details were held on the website and there had been no sign of fraud as a result of the situation.
Anthony McQuiggan, cabinet member for Corporate Services and Information Technology, said: "We take the protection of personal information seriously, and we have acted with immediate effect to address the website error.
"Since we cannot be sure if the records of these individuals have been viewed, we have reported this issue to the ICO, taken immediate actions to secure our databases, and contacted customers proactively to notify them of the issue. No financial data has been compromised."
He added: "We are very sorry that this situation has arisen and apologise for any distress or inconvenience that this may have caused to those individuals affected."