Star Citizen's FPS mode, called Star Marine, has been delayed indefinitely due to an array of technical and gameplay-related issues.

During PAX East in April, developer Cloud Imperium Games released a new video of the mode, which you can see below. It was expected for release later that month, but that never happened.

Star Citizen boss Chris Roberts spoke about at length about the studio's struggles recently as part of his latest "Letter from the Chairman" blog post.

The long and short of it, according to Roberts, is that the current version of Star Marine does not meet the standards that Cloud Imperium Games wanted to achieve.

"There are several issues that will need additional time in order to deliver the first iteration of the gameplay we want you to experience," he said. "The challenges facing the FPS launch are a mix of technical blockers and gameplay issues. The most significant technical hurdle faced today is the networking backend."

Roberts went on to say that Cloud Imperium Games attempted to continue development using "legacy code," but it was later decided to partly abandon this approach--this led to even more issues.

"That meant developing what we're calling a Generic Instance Manager (GIM) and rewriting both the Matchmaker and (for the larger project) the game Launcher from scratch," Roberts explained. "Those efforts are all going well, but they've all taken additional time for our engineers."

Roberts did not provide a timetable for Star Marine's release. However, he did note that only about 15 percent of the overall Star Citizen development team is working on Star Marine. Development on other areas of the game is continuing on schedule as the FPS team works through the issues.

In addition, the Star Marine team is building new characters and weapons, and making "subtle" map changes, alongside its efforts to get the mode up and running, Roberts said.

Hardcore Star Citizen fans will want to read the full blog post for an exhaustive breakdown of the challenges Cloud Imperium Games is facing with Star Citizen's FPS mode.

In other recent Star Citizen news, a new $400 passenger ship is now available to buy.

With close to $85 million in funding, Star Citizen is the most successful crowdfunded game in history.