Almost a million people have stopped paying for World of Warcraft in the last three months.
The sharp drop in subscribers was revealed by Activision Blizzard during talks with analysts about its latest financial results.
At the end of September, WoW had about 10.3 million subscribers, down from 11.1 million at the end of June.
Experts put the decline down to competition from new titles but said WoW was still very resilient.
World of Warcraft's subscriber numbers have been on a steady decline from the peak of 12 million they hit in 2010.
Blizzard's boss, Mike Morhaime, said the biggest decline in player numbers was seen in the Asia Pacific region, with significant falls in China.
He said Blizzard had expected to lose subscribers over the last few months as the keenest players exhausted the fresh content that the Cataclysm expansion added to the game in late 2010.
Despite this, claimed Mr Morhaime, WoW remained the most popular subscription game in the world.
WoW has kept its subscription model even though there has been a general move to free-to-play by its rivals. These include Warhammer, Lord of the Rings Online and DC Universe Online.
Blizzard has gone some way to embracing this change by letting gamers play for free via an account that limits them to WoW's opening sections, caps how much cash they can accumulate, and restricts what they can do with characters.
The game maker has also unveiled ways to make sure veteran players stay involved. Last month it announced players who commit to a new 12 month subscription would get a free copy of Diablo III, early access to the next expansion for WoW, and a flying horse to ride around in the game.
Despite the decline in players of Wow, Activision Blizzard's results were strong with record third-quarter results up 16% over the same period in 2010.
Games journalist John Walker from gaming site Rock Paper Shotgun said WoW had weathered similar dips in the past when there were other strong titles gamers wanted to play.
At the moment WoW faces competition from games such as Arkham City, Skyrim, Battlefield 3 and Activision's own Modern Warfare 3.
"People tend to only want to give their monthly tithe to one game, and so they'll often slip away to see what's new, before slinking back to Warcraft," he said.
But, he added, the fact that millions still regularly played a seven-year-old game showed its resilience.
"If anything, WoW's persistent success with its subscription model is actually bucking any trend that we may be seeing, even if the numbers have dipped slightly," he said.
"While it's offering a sort of faux free-to-play model for its early stages now, there's no doubt that its 10 million plus player-base remains hugely significant."