Ackman, who runs hedge fund Pershing Square Capital Management, sent a letter to his investors saying the FDIC should raise its $250,000 per account limit days after U.S. regulators took over Silicon Valley Bank and Signature Bank, triggering a crisis in U.S. regional banks.

In his annual letter to shareholders he amplified a message he has been blasting for days on Twitter.

"Banking is a confidence sensitive business," and regulators' conflicting public statements have "reduced investor, business, and consumer confidence in our banking system" he wrote.

Taking an individualized, bank-by-bank deposit guarantee approach is a "policy mistake," Ackman wrote, warning this could impair the economy at a time regional banks are instrumental in the real estate and construction loan business.

Several days ago, the man who shares his opinions ranging from the dangers of sugary drinks to how tennis players could earn bigger paychecks, warned on Twitter that the U.S. economy may be headed for a "train wreck."

Last year Ackman, who had waged corporate battles at retailer Target and railroad Canadian Pacific, told investors he would take a "quieter approach" at a time when corporate America knows who he is and that he no longer needed the noisy tactics other corporate agitators do.

This year, he has been tweeting a lot to his 689,800 followers and said in his letter that the social media platform was a "very efficient means to get the message out."

Ackman's investment firm's Pershing Square Holdings portfolio has returned 25.1% per year over the last five years, handily beating its broader stock market index which gained 9.4% a year during the same time.

(Reporting by Svea Herbst-Bayliss; Editing by Josie Kao)

By Svea Herbst-Bayliss