The company said the allegations were that the data breaches had occurred at two of its units in 2012 or earlier.
The data consists of names and e-mail addresses of customers and is available for purchase on the dark web. The dark web is an area of the internet that can only be accessed through software that makes web browsing anonymous.
Optimals NETELLER and Moneybookers units had suffered data breaches as a result of cyber-attacks in 2009 and 2010, but none of its customers lost any money as a result.
Optimal said it had informed the Information Commissioner and the Financial Conduct Authority (FCA) about the matter.
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