Belfast City Council has said an investigation has been launched after personal details of councillors were given to a member of the public.
They include their bank details, dates of birth, national insurance numbers, home addresses, car registration numbers and passport numbers.
The data protection breach was first reported in theIrish News.
The council told the BBC it was "deeply concerned" and had launched an "urgent investigation".
"We are also looking at what legal action we can take to prevent the information being released any further," a spokesperson said.
"The council will take all steps required to protect the personal interests of members and will provide any support or assistance that is required.
"We have launched an urgent investigation into all the circumstances and have also informed the information commissioner."
The Irish News said it had obtained a file on each of Belfast's 51 councillors.
It is believed the confidential document was mistakenly sent via email to a woman in England who had requested contact details for elected representatives.
It is understood the council briefed some councillors on Monday night after the Irish News informed the council about the story.
A spokesperson for the Information Commissioner's Office said: "We have recently been made aware of a possible data breach which may involve Belfast City Council.
"We will be making enquiries into the circumstances of the alleged breach of the Data Protection Act before deciding what action needs to be taken."
The National Association of Councillors, which represents all 580 councillors in Northern Ireland, said it was shocked that "such sensitive information could have been released so carelessly".
"Obviously this information could be seriously misused if it were to fall into the wrong hands," it said.
"We have sought immediate assurances from Belfast City Council that a full investigation is taking place and that measures are being taken to prevent such security breaches in the future and to limit the damage caused by this mistake."