Billionaire investor Carl Icahn, known for pushing for change at the companies in which he invests, has become one of Chesapeake Energy Corp.’s largest shareholders, said a person with knowledge of the matter.

Icahn’s stake in the company may amount to more than 4 percent, said the person, who declined to be identified because the holding hasn’t officially been disclosed. That would make him at least the fifth-largest investor in the second-biggest U.S. natural-gas producer, with a stake valued at more than $400 million based on Chesapeake’s closing price Wednesday.

The presence of Icahn, 76, may add pressure on the board, which is already in the midst of a search to replace Chief Executive Officer Aubrey McClendon as chairman. The shares have lost about a third of their value this year amid a slump in gas prices and revelations about McClendon’s personal borrowings.

Icahn didn’t return phone messages left at his office. Michael Kehs, a Chesapeake spokesman, declined to comment.

Directors agreed to strip McClendon of his chairman title after revelations about his borrowing from firms that do business with the Oklahoma City, Oklahoma-based company. Board members also cut their own pay and started a formal review of the CEO’s financial arrangements.

Two members of Chesapeake’s nine-person board are seeking re-election at the company’s June 8 annual meeting. Shareholders must have owned their stock on April 10 to vote.

A quarterly disclosure of Icahn’s stock holdings as of March 31 showed no Chesapeake shares. However, the filing said Icahn won permission to keep some of his investments confidential.

Icahn’s Fortune

Icahn, whose fortune Forbes magazine estimated at $14 billion as of March, has profited from Chesapeake in the past. He disclosed a 5.8 percent stake in Chesapeake in December 2010. Two months later, Chesapeake announced plans to sell assets and reduce debt, a move Icahn supported. He later sold Chesapeake shares.

“He made, I think, over $500 million and he called me to thank me when it was all over,” McClendon said on a conference call May 14 when asked whether Icahn would again invest in the company. “I have a good relationship with Carl.”

Chesapeake’s biggest disclosed shareholder is Southeastern Asset Management Inc., the Memphis, Tennessee-based money manager run by Mason Hawkins, with a 13.2 percent stake as of May 14, according to data compiled by Bloomberg. Wellington Management Co. had an 8 percent stake as of March 31, and State Street Corp. had 4.1 percent, the data show.

Most money managers have to disclose their overall holdings four times a year. Investors who buy more than 5 percent are required under Securities and Exchange Commission rules to disclose the holding more rapidly.