Consumer champion Martin Lewis has launched a free tool for consumers to use in logging a complaint about discretionary commission agreements (DCAs) in their car loans.

The Money Saving Expert founder highlighted to viewers in his prime time ITV show this week that he's watching the FCA's review of historic DCAs closely and suggested there is no smoke without a fire.

The FCA launched its review after the Financial Ombudsman Service ruled in two cases that lenders' DCAs, combined with a lack of full disclosure to the customer, had put car buyers at a disadvantage and were unfair. The FCA's review hopes to quickly determine if this is a widespread issue.

"My view on this is I don’t believe it would have launched this investigation unless it already had substantial evidence and the likelihood is the investigation now is to make sure it’s got everything right so it can’t be judicial reviewed," said Lewis in his TV show.

"I think it is very very likely it is going to rule that there was seismic, systemic mis-selling."

Lewis added: "A back of the envelope calculation shows this will likely end up being the second biggest ever UK reclaim campaign, after PPI. The average payback will likely be over a grand."

Lewis wants to discourage people from going to claims management companies, which would eat into any compensation that might be awarded, and his Money Saving Expert organisation has now launched a tool to help complainants go through an easy process.

"I think it likely that, when the investigation completes (currently planned to be September), the FCA will set up some type of mass-scale redress scheme – though there's a small chance it'll change its mind and say this is a damp squib. The best way to act is to assume that scheme is coming. 

"This could lead to billions of pounds of overcharged interest paid back to millions of people," says MSE. "Even though there's a pause on firms needing to deal with complaints, its important to get your complaint logged sooner, so there’s less chance you’ll be timed out."

Lewis urged consumers to take action quickly, which has led to motor finance firms and their regulated broker and dealer partners to expect a flood of complaints in the coming weeks.

The FCA recently said it has gone into the review with no prior assumption of what it will find.