Valve has issued violation notices to at least 23 betting sites that use its Steam platform, according to one of the communities in question. CSGO Big, a site dedicated to betting weapon skins in Counter-Strike: Global Offensive, posted the letter from Valve's legal counsel to its Twitter earlier today, writing that it would be "shutting down temporarily" in response.

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"We are aware that you are operating one of the gambling sites listed below," wrote Karl Quackenbash, general counsel to Valve Corporation, in the letter sent out to various communities. "You are using Steam accounts to conduct this business. Your use of Steam is subject to the terms of the Steam Subscriber Agreement ('SSA')."

Valve clarified a week ago that these terms ban external sites from asking users to connect their Steam accounts to trade their in-game items for real money. It also announced that it would begin cracking down on many of gambling sites, ordering them to cease and desist.

The letter orders the sites named to "immediately cease and desist further use of your Steam accounts for any commercial purpose. If you fail to do this within ten (10) days Valve will pursue all available remedies including without limitation terminating your accounts."

Among those listed are CSGO Lotto, CSGO Wild and CSGO Diamonds, each among the most popular gambling communities. CSGO Lotto has been offline since July 8, the day after it was added to a lawsuit filed by a player against several betting sites and Valve, alleging that they facilitated underage gambling. Other sites receiving Valve's letter include CSGO Double, which announced that it was shutting down last week.

Although Global Offensive has been at the forefront of the controversy, thanks to the game's popularity and the involvement of several YouTube personalities in the lawsuit, a Dota 2 site has also been ordered to close.

Kotaku verified with Valve that the cease-and-desist note is real, but we've contacted Valve to clarify whether the 23 domains listed are the only ones asked to shut down thus far. We'll update with more when we receive it.