The issue came to light when angry customers flooded message boards and social media outlets with complaints about mysterious additional charges and/or refunds when purchasing digital currency with a credit or debit card.

While Coinbase initially received flak from users over the unathorised transactions, it then moved on to balme recent changes to the coding of cryptocurrency purchases for the problems.

Some major card networksand issuers recently re-classified merchant category coding for cryptocurrency purchases as cash advances, meaning that banks could hit up customers for additional fees.

"As a result, purchases that occurred between January 22nd, 2018 and February 11th, 2018 may have been refunded and reprocessed — resulting in erroneous charges," says Coinbase in a blog post.

Visa has hit back at the Coinbase allegations: “Visa has not made any systems changes that would result in the duplicate transactions cardholders are reporting. We are also not aware of any other merchants who are experiencing this issue. We are reaching out to this merchant’s acquiring financial institution to offer assistance and to ensure cardholders are protected from unauthorised transactions. Visa does not reverse or resubmit transactions. That is not Visa’s role in the payment system. All transactions are submitted into Visa’s system by a financial institution, which for merchants is their acquiring bank.”

The plot thickens

In a further update posted Saturday, Coinbase offered the following clarification: “Over the last two days, some customers who used a credit or debit card at Coinbase may have seen duplicate transactions posted to their cardholder accounts. This issue was not caused by Coinbase. Worldpay and Coinbase have been working with Visa and Visa issuing banks to ensure that the duplicate transactions have been reversed and appropriate credits have been posted to cardholder accounts. All reversal transactions have now been issued, and should appear on customers’ credit card and debit card accounts within the next few days. We believe the majority of these reversals have already posted to accounts. If you continue to have problems with your credit or debit card account after this reversal period, including issues relating to card fees or charges, we encourage you to contact your card issuing bank. We deeply regret any inconvenience this may have caused customers.”