Altimeter and PAR, which own around 7.1% of United Continental Holdings, have criticised the "underqualified, ineffective, complacent and entrenched" board and said this had caused "inexcusable company underperformance" over the years.
The pair of investors want to bring in new blood and have put forward a number of potential new directors including Gordon Bethune, the former chief executive of Continental Airlines.
United and Continental merged in 2010.
Henry Meyer III, chairman of United, said the company was “deeply disappointed” with Par and Altimeter.
