A laptop computer containing personal details of up to 60,000 people has been stolen from the Citizens Advice Bureau in Belfast.
The data was stored on an advice bureau laptop
The computer was taken from a staff member's car earlier this week.
It holds information on people who have sought advice but the bureau said the data was encrypted and it was highly unlikely that it could be accessed.
The details include bank account numbers, national insurance numbers, names, addresses and dates of birth.
Northern Ireland CAB spokesman Derek Alcorn has issued an apology and said the organisation would be writing to all the individuals concerned.
"We can say that the data on the computer is protected by three levels of security, including a high level of encryption," he said.
"It is highly unlikely that a criminal will be able to access the data, but people who have used Citizens Advice in the Belfast area should check for anything unusual.
"We believe the information, even if it were accessed, would not on its own allow access to a bank account because banks require other security information and passwords."
He said a helpline had been established and would be staffed over the weekend from 1000 GMT until 1600 GMT.
"Information is also on our website," he said.