The Institute of Directors has warned there is no "check on Mike Ashley's power" at Sports Direct as the company prepares for a showdown with investors over a controversial share bonus scheme.

"The fact that the board of Sports Direct is attempting once again to push through a hugely generous pay award for Mike Ashley – in the face of shareholder opposition – suggests weak governance at the company," said Dr Roger Barker, director of corporate governance at the Institute of Directors (IoD).

On Wednesday shareholders will vote on a fourth attempt by the company to pay a share bonus to Ashley, who owns 58% of the company. The latest plan is linked to a company-wide scheme that would hand members 25m free shares – worth more than £180m at the current price – if the firm doubles earnings by 2019. But Sports Direct has angered investors by refusing to spell out how many of the shares Ashley is entitled to.

"This type of pay package would be unthinkable for a senior executive who was not also the company's major shareholder," added Barker. "It raises doubts about whether the board is acting as an effective independent check on Ashley's power."