Popular Bitcoin exchange Coinbase has been overcharging users thousands of dollars, blaming a bug linked to how credit card companies bill for the service.

Coinbase said it has fixed the problem which saw customers charged multiple times for transactions, while it has promised refunds to those affected.

Multiple customers complained on user forums that the app, which allows people to buy and sell cryptocurrencies like Bitcoin and Ethereum, had been charging them multiple times for transactions.

Coinbase put the error down to changes in MMC code, the code used by Visa to identify credit and debit card transactions. The Visa changes came as multiple banks said they would no longer allow users to buy cryptocurrencies using credit cards.

"Coinbase will ensure that each affected customer will be refunded in full for any erroneous charge," the company said. "We deeply apologize for any frustration this may cause. We are actively working with banks, processors and networks to improve the digital currency purchasing experience."