AmTrust Financial Services Inc. jumped almost 12 percent in late trading Thursday after billionaire Carl Icahn disclosed a stake in the company and said he planned to oppose its plans to go private.

“I am strongly against the proposed going-private transaction and intend to solicit proxies against the deal,” Icahn, who said he owns almost 9.4 percent of the insurer, wrote in a letter to AmTrust’s board.

In March, Stone Point Capital, the Karfunkel family and AmTrust’s Chief Executive Officer Barry Zyskind agreed to buy the company in a deal that valued the firm at about $2.7 billion, after sweetening their January bid by 10 percent.

Icahn said the proposed “squeeze out” transaction fails to take into consideration most of AmTrust’s minority holders. The board “stealthily” set a date of April 5 for shareholders of record, who will be allowed to vote on the matter at a meeting on June 4. The cut-off date wasn’t disclosed to shareholders or the market, Icahn said.

“Going-private transactions are rarely without controversy but the law is clear: non-controlling shareholders must be treated fairly, both in terms of process and price,” Icahn wrote, adding that the proposed transaction falls short of that.

Icahn said he is in discussions with his lawyers and is assessing his options.

A spokeswoman for New York-based AmTrust didn’t immediately return a message seeking comment.

The takeover offer came after a rocky period for AmTrust, marked by financial restatements, troubles in accounting controls and reports of an investigation by the U.S. Securities and Exchange Commission. The family of Zyskind and Director George Karfunkel agreed last year to inject $300 million into the firm in a private placement deal.

The insurer has worked to stabilize the company, naming a new chief financial officer last year.