The bank admitted it had breached the Consumer Credit Act by wrongly charging interest to personal loan customers due to a glitch relating to arrears notices and statements, according to the Daily Mail.
The problem, which dates back as far as 2008, affected up to 300,000 customers and could lead to a potential £100m payout, however the full extent of the issue is not yet known.
The bank has said similar problems may have occurred with its Barclaycard business, which could add a further 10 million UK customers to the mix.
Barclays said it would contact those affected by the glitch.
The news came after it was reported that the bank was in negotiations with the regulator over a potential £50m fine for breaching listing rules back in 2008.
Barclays had allegedly failed to disclose details of a £11.8bn capital-raising from Qatari and other sovereign investors at the time.
Meanwhile the bank faced regulatory fines of £291m for its role in the manipulation of LIBOR and a contested $453m fine for rigging US energy markets.
It announced at the end of July that it planned to raise £5.8bn from investors to shore up its capital position, the preliminary prospectus for which is expected to be published next week.
