Martin Sullivan under fire from disgruntled shareholders.

AIG chief executive Martin Sullivan has come under renewed pressure after the insurer revealed it was under investigation by the US regulator over the valuations of its sub-prime related assets.

Sullivan’s position, already considered precarious following record losses over the past two quarters and $20bn in write-downs, has been further undermined by letters sent to the board from disgruntled shareholders.

This includes former chief executive Hank Greenberg, who recently described the company as being in a state of crisis. Greenberg controls around 12% of the company.

Analysts suggested that Sullivan would not survive a third successive large loss.

The AIG board said it would continue to back its chief executive. In a statement the insurer said Sullivan would “successfully navigate the organisation through these challenging times”.

It added: “He has the experience to know that certain legacy committees and oversight bodies were ineffective and created new mechanisms to provide better oversight.”