Google’s YouTube has reached a preliminary deal to buy Twitch, a popular videogame-streaming company, for more than $1 billion, according to sources familiar with the pact.

The deal, in an all-cash offer, is expected to be announced imminently, sources said. If completed the acquisition would be the most significant in the history of YouTube, which Google acquired in 2006 for $1.65 billion. The impending acquisition comes after longtime Google ad exec Susan Wojcicki was named CEO of YouTube earlier this year.

Reps for YouTube and Twitch declined to comment.

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San Francisco-based Twitch lets users upload and watch free, live gameplay videos that can be streamed from Microsoft Xbox One and Sony PlayStation 4 consoles, and also sells a $9 monthly ad-free subscription and subscriptions to individual channels for about $5 per month. The company claims to have more than 45 million monthly users, with more than 1 million members who upload videos each month. It also has deals to distribute shows from partners including CBS Interactive’s GameSpot, Joystiq and Destructoid.

YouTube is preparing for U.S. regulators to challenge the Twitch deal, according to sources. YouTube is far and away the No. 1 platform for Internet video, serving more than 6 billion hours of video per month to 1 billion users worldwide, and the company expects the Justice Department to take a hard look at whether buying Twitch raises anticompetitive issues in the online-video market.

Twitch was launched in June 2011 by Justin Kan and Emmett Shear, co-founders of Justin.tv, one of the first websites to host livestreaming user-generated video. Shear currently serves as CEO of Twitch.

Founded in 2011, the startup has raised about $35 million in funding. Investors include Bessemer Venture Partners, Alsop Louie Partners, WestSummit Capital, Take-Two Interactive Software, Thrive Capital and Draper Associates. Twitch Interactive, which includes Justin.tv, has about 130 employees.

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Gameplay videos already are a highly popular category on YouTube. Twitch also plays in the separate “e-sports” market covering live professional gaming competitions; others in that space include tournament organizer Major League Gaming, a New York-based company whose investors include Oak Investment Partners and Relativity Media CEO Ryan Kavanaugh.

In March 2014, Twitch represented 1.35% of all downstream bandwidth on North American fixed-access broadband networks — nearly triple from last fall, according to bandwidth-equipment company Sandvine.