FleetCor Technologies, Inc. has collected "hundreds of millions" in improper and hidden fees from its fleet customers since at least 2014, according to a civil complaint filed by the Federal Trade Commission against the Atlanta-based payment processor.

The lawsuit came Dec. 20 following a breakdown in negotiations between the company and commerce regulator on Dec. 19, when the company told MarketWatch talks were "at an impasse." FleetCor said the FTC's claims are "without merit" and the impasse was due to "unreasonable demands for redress made by the FTC."

The complaint accuses FleetCor and its CEO Ronald Clarke of violating the FTC Act's prohibitiions on unfair and deceptive acts and practices.

Specifically, the complaint alleges that customers haven't achieved FleetCor's advertised per-gallon savings. On average, customers have saved a fraction of a cent per gallon instead of the 5 to 10 cents per gallon touted by the company. In addition, the fees charged by FleetCor "have exceeded any savings otherwise obtained using FleetCor's cards."

The complaint was filed with U.S. District Court for the Northern District of Georgia. If the agency wins the case, FleetCor could be made to pay redress in the form of a fund with proceeds that would be paid out to customers.

At least tens of thousands of customers have complained to the company, government agencies, and the Better Business Bureau, according to the FTC.

Read the full FTC complaint here.