Icahn in a letter to AmTrust's board said he strongly opposed the going-private deal, which was announced by the company on March 1.

The company's shares jumped more than 10 pct in after-market trading after the stake was disclosed.

Icahn also said that he wants AmTrust to change the date for the special meeting, which was originally scheduled for June 4.

AmTrust said on March 1 it would be acquired in a $2.7 billion deal by a group of shareholders including its founding family, chief executive officer and private equity funds.

(Reporting by Parikshit Mishra in Bengaluru; Editing by Shounak Dasgupta)