Update: Riot has been in touch with VideoGamer.com to challenge DFC's report, telling us that the original press release promoting the report has been retracted.
In addition, the League of Legends firm told GamesIndustry that the title sees "over 500,000 peak concurrent players every day on just the EU West shard." This figure doesn't include players in the US.
Valve's Steamgraph shows an all-time user high for Dota 2 of only 325,879.
"You can see how we would have some issues with the accuracy of this report," said the Riot Games spokesperson.
Original: Valve's Dota 2 was the most played "core" PC game in Q1 2013 by western gamers, overtaking Riot's League of Legends, GamesIndustry reports.
A report from DFC Intelligence on PC gaming in North America and Europe not only shows that Dota 2 was the most played game, but that both it and fellow MOBA League of Legends also proved to be much more popular than Blizzard's World of Warcraft.
"Usage of League of Legends has been steady the past year and surprisingly the huge success of Dota 2 did not seem to cut into its popularity as much as would be expected," said DFC analyst Jeremy Miller.
Overall, PC gaming was up 22 per cent when compared to the first quarter in 2012, with DFC pointing to a number of blockbuster titles.
"The past year has seen numerous blockbuster PC games that have really helped drive usage. This includes among others Diablo III, Guild Wars 2, Battlefield 3, Minecraft, World of Tanks and Valve's Counter-Strike series," added DFC analyst David Cole.
Casual and browser-based games are not included in the report.