Institutional investors in ExxonMobilhave added their voices to the campaign for the world's biggest private sector company to separate the roles of chairman and chief executive.

Nineteen investors, including Calpers and Calstrs, the powerful Californian retirement funds for public employees and teachers, have backed the resolution calling for the appointment of a new chairman, as well as four resolutions calling on Exxon to do more to address the threat of climate change.