Carried out by consumer group Which?, the investigation found lenders appear to be breaking guidelines that are designed to safeguard the private financial details of customers.
The organisation noted the Information Commissioner's Office (ICO) - which received 1,163 complaints about banks and building societies in just one year - is not doing enough to hold institutions to account for their actions.
Which? observed that lenders hold sensitive data - and customers should be able to trust them with it.
"We have made some shocking discoveries about the number of breaches they could be committing and the scant action the ICO has taken about potentially serious lapses," the group stated.
Moody's Investors Service recently revealed it is considering downgrading the status of 14 banks in the UK, including Lloyds Banking Group and the Royal bank of Scotland Group.
By Asim Shah