The future of OpenAI is in jeopardy after more than 700 of its 770 employees signed a letter on Monday saying they may leave the company for Microsoft if the ousted chief executive, Sam Altman, is not reinstalled at the high-profile artificial intelligence start-up.

OpenAI’s four-person board shocked the tech industry early Friday afternoon when it removed Mr. Altman, saying they could no longer trust him. One of the board members who pushed out Mr. Altman then reversed course on Monday and signed the letter demanding that he be reinstated.

The decision by the board set off a frantic weekend of unexpected corporate jockeying that ended with Mr. Altman joining Microsoft to start a new A.I. project. By early Monday morning, the 700 employees had signed the letter, according to three people familiar with the matter.

The upheaval leaves the future of one of the fastest-growing companies in Silicon Valley history in doubt. At a time when the industry was reeling in the wake of mass layoffs, OpenAI’s technology fueled the creation of hundreds of start-ups. Now, many of those businesses are concerned about their prospects.

“This is the debacle of the decade,” said Gaurav Oberoi, the founder of Lexion, a start-up that relies on OpenAI to help companies streamline legal, sales and vendor contracts. “It’s a lesson in how to destroy a huge amount of value overnight and their own reputation.”

OpenAI declined to comment. Emmett Shear, whom the board named as interim chief executive late on Sunday, declined to immediately comment because he was busy on another call.

The letter said that Microsoft had assured OpenAI employees that there were positions for them all if they chose to join its new A.I. subsidiary. Microsoft declined to comment.

Signatory No. 12 on the letter is a surprising name: the OpenAI chief scientist, Ilya Sutskever, a member of the four-person board that fired Mr. Altman. “I never intended to harm OpenAI,” he said on X, formerly known as Twitter. “I love everything we’ve built together and I will do everything I can to reunite the company.” Mr. Altman reposted the message and added three red hearts.

Mr. Sutskever did not respond to a request for comment.

In addition to Mr. Altman, several key OpenAI employees have already joined Microsoft’s new A.I. subsidiary. This includes Greg Brockman, the OpenAI president who quit the start-up in solidarity after Mr. Altman was ousted. Early Monday morning in a post to X, Mr. Brockman said that he and Mr. Altman would also be joined at Microsoft by three OpenAI researchers: Jakub Pachocki, Szymon Sidor and Aleksander Madry.

Mr. Pachocki led the development of GPT-4, the technology that underpins OpenAI’s popular chatbot, ChatGPT. He has long worked closely with Mr. Brockman, an engineer who helped found OpenAI in 2015 alongside Mr. Altman and has been deeply involved in almost all aspects of the company’s operations from its earliest days.

OpenAI staff was in upheaval in the hours after the board posted its memo on Friday night, two OpenAI employees told The New York Times. Employees were privately sharing morbid jokes and memes about the power struggles from the HBO show “Succession,” the two said. Many used private group messaging chats and video calls to plan their next steps — and to commiserate with one another.

OpenAI still retains a partnership with Microsoft. Satya Nadella, Microsoft’s chief executive, said in an early Monday post to X that his company remained committed to it. He said that Microsoft would continue to work with the start-up to sell a wide range of products and services based on GPT-4 and other OpenAI technologies.

But if most OpenAI employees leave for Microsoft, the start-up will have difficulty building the next generation of A.I. technologies — systems that will be more powerful than ChatGPT. Others companies, including Google and Meta, are working on such technologies.

Mr. Oberoi of Lexion said that his company had been using OpenAI’s large language models, or L.L.M.s, to develop new features because its A.I. technologies are more advanced than any others in the market. But in the wake of this weekend’s turmoil, he said that Lexion will be developing parallel features with Anthropic, an OpenAI rival, so that the company “can switch quickly if need be.”

“This underscores a big discussion happening: Are you going to build your technology and platforms and key features on third party L.L.M.s?” Mr. Oberoi said. “As a builder on top of their products, I worry if there will be any other sudden decisions that could impact our models. Also, it’s really expensive.”

Late Monday morning, Mr. Altman made an effort to appease the customers of OpenAI. In a post to X, he said that the top priority for Mr. Nadella and himself was to ensure that OpenAI continued to thrive. “We are committed to fully providing continuity of operations to our partners and customers,” he wrote.

“We are all going to work together some way or other, and I’m so excited,” he wrote in another post to X. “One team, one mission.”