A long-simmering public dispute between Facebook and Apple has neared a boiling point.

With the aid of outside legal counsel, Facebook for months has been preparing an antitrust lawsuit against Apple that would allege the iPhone-maker abused its power in the smartphone market by forcing app developers to abide by App Store rules that Apple’s own apps don’t have to follow, according to two people with direct knowledge of Facebook’s efforts.

The legal preparations by Facebook signal that the feud between the companies could further escalate, though ultimately Facebook may decide not to file a suit. Its executives are facing internal resistance from some employees over its public campaign against Apple, a fight that recently has centered on a change to iPhone software that will make it harder for Facebook and its advertisers to track people across apps. Now Facebook CEO Mark Zuckerberg is attempting to build a broad legal case arguing that Apple’s rules for app developers—which force them to use Apple’s in-app payment service, for instance—make it harder to compete against Apple in areas such as gaming, messaging and shopping.