Activist investor Elliott Investment Management on Monday warned of a proxy fight to push for leadership changes at budget carrier Southwest Airlines.

In a letter to Southwest’s board, Elliott said while it is open to collaboration, it plans to “move expeditiously to give shareholders a direct say on the necessary leadership changes.”

Elliott, known for pushing changes to boost shareholder returns, has called for areconstitutionof Southwest’s board with “new, truly independent” directors possessing expertise in airlines, customer experience and technology.

The activist investor has also called for a leadership change citing Southwest’s disappointing financial results.

In a statement, Southwest said it remains open to constructive conversations with Elliott, including evaluating additional “strong and independent” director candidates. The airline, however, added the investment firm has focused on personal attacks on its leadership and board, conditioning any serious discussions on an immediate CEO change.

In its letter, Elliottsaid the airline’s performance is “unacceptable” and a leadership change is required to return it to its once-leading position in the industry.

“Shareholders simply do not believe this board and management team are capable of devising and executing a bold new plan to turn around Southwest,” it said.

Elliott currently owns an 11% stake in the company, making it one of the largest investors.

Last week, Southwest said the activist investor had made filings with antitrust authorities that would allow it to acquire an even larger voting power as early as July 11. It prompted the company to adopt a “poison pill,” used by corporate boards to thwart hostile takeover bids, to deter Elliott from raising its stake.

Elliott said it is not seeking control of Southwest, but added the airline’s board has put its own “self-interest ahead of the company’s” by adopting the “poison pill.”

It also criticized the appointment of aviation industry veteran Rakesh Gangwal to Southwest’s board, calling it an attempt on the part of the board to “entrench itself and the current management team.”

“These actions…demonstrate how profoundly out of touch Southwest’s board has become with shareholder sentiment and with the reality of the situation,” Elliott said in its letter.

Gangwal – who co-founded InterGlobe Aviation, the operator of India’s largest airline IndiGo – has also served on the boards of US Airways Group, CarMax and Office Depot, among others.

“Rakesh’s expertise in travel technology will be valuable as we continue to make investments that support our operations and strategic initiatives,” said Gary Kelly, executive chairman of Southwest’s board.