Platinum Gaming has been fined £1.6m ($2m) by the Gambling Commission for social responsibility and money laundering failures.
Owned by Kindred Group, Platinum Gaming provides the license for Unibet’s operations.
The Gambling Commission launched an investigation after receiving reports a convicted fraudster spent £629,420 of stolen money with Platinum Gaming.
The Commission’s enquiries revealed the customer’s deposits were so high and losses so significant Platinum Gaming should have considered refusing or barring service. They also showed the operator failed to enquire about the customer's source of funds.
Gamesys was recently fined £1.2m for similar offences.
Kindred Group acknowledged and accepted the punishment and said it deeply regrets the incident.
A statement from the operator read: "As a group, we monitor, act on and report all suspicious behaviour we detect, which was done in this specific case. However, in this specific case we did not act quickly enough.
"Kindred’s processes and procedures, along with the number of reported cases, are documented in the Group’s 2018 Sustainability Report."
Richard Watson, Gambling Commission Executive Director, said: "There were weaknesses in Platinum Gaming’s systems and as a consequence, more than half a million pounds of stolen money flowed through the business.
"This is not acceptable and I would urge all operators to carefully read this case and learn lessons so they don’t make the same mistakes."