The DoJ is looking into how Apple has used its power over its gadgets and software to make it hard for customers to switch to other devices and for rivals to compete.

They have noted how the Apple Watch works better with the iPhone than with other phones and how Apple blocks competitors from its iMessage service. They have also eyed up Apple’s payments system for the iPhone, which stops other money firms from offering the same services.

All this is leading up to the most significant US lawsuit against Apple, which is for now the world’s most prosperous tech company. US regulators will have sued four of the biggest tech companies for unfair business practices in less than five years if the lawsuit goes ahead.

The Justice Department is battling Google in two lawsuits, focused on its search and ad tech businesses. At the same time, the Federal Trade Commission has sued Amazon and Meta for crushing competition.

The Apple suit would likely be even more significant than previous cases against the company, hitting its “walled garden of delights” business model that ties the iPhone with gadgets like the Apple Watch and services like Apple Pay to lure customers hooked to its products.

Rivals have said they have been locked out of crucial Apple features, like the Siri voice assistant, making them claim the practices are anti-competitive.