Troubled software and engine firm Crytek has filed a lawsuit against Cloud Imperium Games and Roberts Space Industries saying that the latter is infringing copyright and in breach of contract.

As reported by PCGamesN, the court documents were filed on Tuesday, with Crytek claiming that the makers of Star Citizen and their wrong-doings "have caused substantial harm to Crytek".

RSI has given a statement, again to PCGamesN, saying that the lawsuit is meritless

"We are aware of the Crytek complaint having been filed in the US District Court," CIG say in their statement.

"CIG hasn’t used the CryEngine for quite some time since we switched to Amazon’s Lumberyard. This is a meritless lawsuit that we will defend vigorously against, including recovering from Crytek any costs incurred in this matter."

So what's the problem here? The Star Citizen developers teamed up with Crytek to prepare promotional materials for the crowdfunding campaign that pushed the sci-fi game to the big leagues. This relationship was later formalised after the Kickstarter push with a Game License Agreement which Crytek says was done at a below-market price.

This is where the bulk of the legal disagreement appears to come from - as part of this agreement, CIG and RSI agreed to use CryEngine exclusively. Star Citizen is now using Amazon Lumberyard, which itself is licensed and based off CryEngine as of 2015.

This agreement also extended to only one game, too, so when CIG announced the Squadron 42 spin-off, which is going to be available as its own game, Crytek says the contract has been broken.

And if all this wasn't enough, Crytek also alleges that its copyright notice - i.e. that legal blurb and logo you see at the end of a trailer - was meant to be included in marketing materials. RSI and CIG neglected to do this, with Crytek saying that this was part of the discount-rate on the license fee: the amount of promotion there was potential for.

It continues - Crytek's final complaint is to do with the license agreement saying they will not share CryEngine tech to third-parties, something it says the Star Citizen developers are doing with its BugSmashers line-up of videos where the game makers talk about how they are fixing issues with Star Citizen.

Development on Star Citizen started back in 2011 before it took to Kickstarter. Then the game had a release date for 2014, before being bumped back to 2016. To date it has raised more than $168.5m from its community, is selling digital land to gamers, and does not have a release date.

Meanwhile, Crytek has found itself in constant financial problems, having to sell off Crytek UK to Deep Silver - who renamed the studio Dambusters - before having to close a number of studios last year amid more financial problems.

That hasn't stopped the company teaming up with a cryptocurrency called CryCash as its launch partner.