Ubisoft has confirmed that the game keys it deactivated last week were purchased through Origin with a stolen credit card then resold via third-party websites.

Ubisoft deactivated thousands of keys for games including Far Cry 4, Assassin's Creed: Unity, Watch Dogs, and The Crew, leaving many gamers without access to games they had purchased from sites such as G2Play.

Many of the keys were sold by smaller resellers through an eBay-style key marketplace called Kinguin.

Kinguin said it had received over 4600 customer tickets in the last 72 hours from customers demanding refunds.

It estimates that 148,377 euros (NZ$225,857) will be refunded as a result.

According to Kinguin, an unidentified Russian approached many of its merchants with the keys, and 35 of its 4000 or so merchants purchased them for resale.

"These merchants now claim that their 'source' disappeared and that they were left hanging,” wrote Kinguin marketing head Bartłomiej Skarbiński on the Kinguin website.

“All merchants with no exception declared full will to cooperate and refund all affected customers. We as Kinguin would like to thank them for that."

Kinguin estimates that tens of thousands of keys could be affected, and was puzzled that such a large quantity of keys could be purchased from Origin without anyone being alerted.

"We at Kinguin do not claim ourselves technologically more advanced than Ubisoft or Origin however we do verify big or unusual purchases," Skarbiński said.

"We believe these platforms must have access to anti-fraud e-commerce tools that should raise alarm flags in such cases."

He also admitted that merchants needed to be more careful too.

"Merchants must pay bigger attention to who their business partners are and avoid risk transactions with new entities," he said.

In a statement to Eurogamer, Ubisoft strongly recommended that players purchase keys only from the Uplay Store or trusted retailers.

"We regularly work with our authorised resellers to identify and deactivate fraudulently obtained and resold keys," said an Ubisoft spokesperson.

"In this case, we confirmed activation keys were recently purchased from EA's Origin store using fraudulent credit card information and then resold online. These keys may have been deactivated. Customers who may have been impacted should contact the vendor where they purchased the key for a refund."

Earlier, Skarbiński had called Ubisoft's actions "unfair towards the players".

"From the gamer point of view its like going out to the store, purchasing a copy of the game, taking it home and suddenly a knock-knock on the door with Ubisoft representative taking the copy away - not even asking you as a paying customer to return it," he said.

"We believe Ubisoft had no legal basis for its action. They did it just because they simply can. Kinguin of course is not going to challenge Ubisoft in court as we are not match up for these giants. We will continue to focus on customer's satisfaction and our customers know we have never let them down."

He later vowed to "help everyone affected by this unpleasant situation".