Read the letter: Twitter accuses Microsoft of using its data in unauthorized ways

Twitter is accusing Microsoft of using the social media company's data in ways that were unauthorized and never disclosed.

Alex Spiro, a partner at Quinn Emanuel Urquhart & Sullivan and attorney for Twitter owner Elon Musk, sent a letter to Microsoft on Thursday laying out the claims, including that the software company "may have been in violation of multiple provisions" of its agreement with Twitter over data use.

It's the latest rift among tech companies in the growing debate over who owns data that can be used to train artificial intelligence and machine learning software. The New York Times first reported on the letter, a copy of which was obtained by CNBC.

After Musk led a buyout of Twitter in October and appointed himself CEO, the company started charging for use of its application programming interface, which enables developers to embed tweets into their software and services and access Twitter data.

The API was previously free to use for some researchers, partners and developers who agreed to Twitter's terms. Twitter API-driven apps include Hootsuite, Sprout Social and Sprinklr.

According to the letter from Spiro to Microsoft CEO Satya Nadella and the company's board, last month Microsoft "declined to pay even a discounted rate for continued access to Twitter's APIs and content."

As of April, Microsoft had at least five products that used the Twitter API, including the Azure cloud, Bing search engine and Power Platform low-code application development tools, Spiro wrote.

The agreement restricts excessive use of Twitter's programming interfaces. However, for one of the Microsoft services using Twitter data, "account information outright states that it intends to allow its customers to 'go around throttling limits,'" Spiro wrote.

A Microsoft spokesperson acknowledged receipt of the letter and told CNBC the company will review it and "respond appropriately."

"Today we heard from a law firm representing Twitter with some questions about our previous use of the free Twitter API," the spokesperson said in an email. "We look forward to continuing our long-term partnership with the company."

Musk has been openly critical of Microsoft's tight relationship with OpenAI, the creator of the chatbot ChatGPT. Musk was an early backer of OpenAI, but the company has since raised billions of dollars from Microsoft, which is embedding its AI technology into many core products.

"Microsoft has a very strong say, if not directly controls, OpenAI at this point," Musk told CNBC in an interview this week. Nadella recently challenged Musk's claim in an interview with CNBC's Andrew Ross Sorkin, saying Microsoft has "a noncontrolling interest" in the startup.

Spiro did not name OpenAI or mention its ChatGPT and DALL-E applications or large language models in the letter. He did press Microsoft for any details about, "a description of any token pooling implemented in any of the Microsoft Apps, including the time period(s) when any such token pooling occurred and the number of tokens that were pooled."

Musk and Nadella have had other interactions of late.

Last year, Musk approached Nadella as he was raising money for his Twitter buyout, according to text messages that became public via court filings. Nadella wrote in one text to Musk, "will for sure follow-up on Teams feedback!" Teams is Microsoft's chat app.

Read the full letter from Twitter to Microsoft, here.