The British financial regulator says the Presbyterian Mutual Society in the North broke the law in the way it was being run.
Thousands of small savers risk losing the bulk of their money following a decision earlier this year to put the society into administration.
The organisation got into trouble last year when the British government announced a guarantee scheme for people with bank savings.
This prompted savers to move their money out of the Presbyterian Mutual Society as it was not covered by the guarantee.
The British Financial Services Authority now says the society was effectively being run as a bank without the necessary authorisation, but does not say that this caused the organisation's collapse.
It also says it won't be taking a case against any of those involved in the society.