The Financial Ombudsman Service may reopen thousands of completed cases following undercover reporting that discovered staff had incomplete training.
Some staff had to resort to using Google to do their own research on complained-about products.
Other employees said they were thrown into cases with little or no training.
The findings air tonight in Channel 4’s Dispatches, but have been leaked to The Times.
Dispatches sent an undercover reporter to train as an investigator at the FOS.
One FOS staffer said the financial watchdog had set tough targets for employees, leading to decisions being churned out or sidelined due to workload.
The Times’ report says another employer said ruling against consumers was seen as the easy option.
A FOS spokeswoman told The Times that the Dispatches report does not represent the watchdog at its best. She added that FOS staff are “committed to doing the right thing” and the body always wants to improve.
Last October the FOS announced plans to hire more staff after cutting hundreds in 2016.