OpenAI faced a growing revolt among staff and investors calling for the resignation of three directors after a botched boardroom coup ousted chief executive Sam Altman from the world’s leading artificial intelligence company.

Employees said in a letter to the board that the directors had “undermined our mission and company” by the way they fired Altman and his co-founder Greg Brockman on Friday. The tally of signatures has increased to about 700 of OpenAI’s 770 staff by Monday afternoon, according to employees posting on social media site X. OpenAI did not respond to confirm the figure.

After abortive talks to reinstate Altman on Sunday, in which he demanded the board resign as a price for his return, OpenAI’s board turned instead to Emmett Shear, co-founder of video-streaming service Twitch, as interim CEO. Microsoft, the software company that is also OpenAI’s biggest investor, announced it had hired Altman and Brockman to head a new AI unit.