The unannounced sequel is said to have undergone multiple revisions during what is described by a "reliable source close to Valve" as a "chaotic" development process.
© Valve
The game is rumoured to contain RPG elements, as well as "open-world quests and NPCs", reports French website Journaldugamer (via International Business Times).
Initially conceived as a pure first-person shooter, the development team have supposedly added puzzle solving elements and exploration.
Rumors surrounding the development of Half Life 3 and Half Life 2: Episode 3 began with the 2007 release of Half Life 2: Episode 2.
Valve co-founder Gabe Newell quashed rumors of a Half Life 3 reveal at E3 2010, while the company has failed to make an appearance at the past two E3 events in Los Angeles.
The game was subsequently tipped for a gamescom reveal, but failed to materialise.
Earlier this year, a Valve employee was spotted wearing a Half Life 3 t-shirt, and a voice actor revealed his role in the highly-anticipated sequel.
More recently, a collection of alleged Half Life 2: Episode 3 concept images were leaked online.
Last week marked the release of Black Mesa, a fan-made recreation of the original Half Life.
Picture gallery - alleged concept art for Half-Life 2: Episode 3 below:
Copyright: Valve
