Twitter has threatened to sue Meta over its new Threads app, which Mark Zuckerberg has openly billed as a rival, claiming the company has violated Twitter’s “intellectual property rights”.

In a letter to CEO Mark Zuckerberg, first published by the news outlet Semafor, a lawyer for Twitter said the company “has serious concerns that Meta Platforms (Meta) has engaged in systematic, willful and unlawful misappropriation of Twitter’s trade secrets and other intellectual property”.

“Twitter intends to strictly enforce its intellectual property rights, and demands that Meta take immediate steps to stop using any Twitter trade secrets or other highly confidential information,” Alex Spiro wrote in the letter.

Meta launched Threads, a text-based conversation app intended to rival Twitter, on Wednesday to a largely positive reception. The company said Threads garnered 30m sign-ups in less than 24 hours after launching, apparently making it the most rapidly downloaded app ever. Threads accounts are linked to Instagram profiles, making the process to sign up seamless between apps and giving the Twitter copycat a built-in user base.

“With that knowledge, Meta deliberately assigned these employees to develop, in a matter of months, Meta’s copycat ‘Threads’ app with the specific intent that they use Twitter’s trade secrets and other intellectual property in order to accelerate the development of Meta’s competing app, in violation of both state and federal law as well as those employees’ ongoing obligations to Twitter,” the letter reads.

“Competition is fine, cheating is not,” Musk tweeted on Thursday.