Ubisoft has acknowledged a serious Assassin's Creed 3 bug which is wiping users' save data.

The error occurs when users download this week's Hidden Secrets DLC, a compilation of the various pre-order bonus levels and outfits.

A number of players have experienced main mission or side mission progress being reset, with the possibility of countless hours of game time lost.

"We're able to reproduce this issue at the studio and are working on a fix/workaround," Ubisoft Montreal community developer Gabe Graziani advised fans on the official Ubisoft forum.

"From what we can tell, this problem seems to be rare and should generally only affect a very small number of players. That said, we're taking the speedy and complete resolution of this very seriously."

The bug occurs when players download the DLC while playing the game. This is, right now, something that will affect a number of fans as the DLC is only available to Season Pass subscribers through the in-game store (it's not on Xbox Marketplace, at least).

Ubisoft issued the following advice:

"AVOIDING THIS BUG: Do not load your saved AC3 game while the DLC is downloading in the background. This appears to be the key trigger for this bug. You can do anything else you want to do, just don't load up your saved game and play it while the DLC is downloading."

The add-on will be released for non-Season Pass holders next week.