Twitter's stock price rose 27 percent once Musk belatedly disclosed his stake, the lawsuit said. "During the period that Musk was required to publicly disclose his beneficial ownership but had failed to do so, he spent more than $500 million purchasing additional shares of Twitter common stock," it said.

Musk, who bought Twitter in October 2022 and renamed it X, had started amassing shares earlier that year. His ownership amounted to more than 5% by March, requiring him to file a disclosure report. The SEC says the report was 11 days late, allowing Musk to "underpay by at least $150 million” for shares purchased before he disclosed his stake.