Less than two-thirds (65%)of payday loans are fully paid back on time or early, research for the Competition Commission has found.
The Commission, which is carrying out an industry-wide probe following reports of "deep-rooted" problems in the payday sector, has published a progress report.
The report said that repeat use of payday loans, either by rolling one over or taking out another loan, is "prevalent". It has also found seven out of 10 customers reported that they had not shopped around before taking out their most recent loan, and six out of 10 said they had never done so.
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Looking at "repeat customers", the Commission said around half of customers who have never taken out a loan with a given lender before either end up either rolling over their first loan or borrowing more money from the same lender within 30 days of the original loan.
The Commission found that the biggest players in the sector - Wonga, DFC Global Corporation and Cash EuroNet - account for most loans issued.
The Commission is considering whether firms would find it harder now to enter the market or expand than when the major lenders did.
From April, new regulator the Financial Conduct Authority (FCA) will start to oversee payday firms.
The FCA has announced plans to crack down on the sector, including limiting the number of times payday lenders are allowed to roll over loans to twice, forcing them to put "risk warnings" on their advertising and limiting the number of attempts lenders can make to claw back money if there is insufficient cash in a borrower's bank account to two.