The organisation slammed the “shocking” findings, which revealed that firms were penalising loyal customers.
Citizens Advice said the situation was “symbolic of a wider issues” across a number of essential markets, including mortgages and savings.
The body called for tougher measures to combat the problem, including “enforcement action.”
Gillian Guy, chief executive of Citizens Advice, said: “We knew insurance companies were penalising loyal customers, but it is shocking that the FCA has found many companies don’t even seem to have oversight of how much they’re charging customers.
“The insurance industry has said they want to tackle the loyalty penalty – but it’s hard to see how they can achieve this when companies don’t even have these basics in place.
“The FCA is treating the loyalty penalty seriously by conducting this review and scrutinising firms through a market study.
“We now expect strong regulatory action to stamp out this problem and enforcement against firms who are overcharging loyal customers.”
She added: “This is symbolic of a wider issue across essential markets like mortgages, broadband, mobile and savings.
“The Competition and Markets Authority needs to look very carefully at this as they respond to our super-complaint on the loyalty penalty.”