Tim Cook

Institutional Shareholder Services (ISS) said it has "significant concerns" over the size of the award, up from $14.8m the year before.

Mr Cook, whose net worth is reportedly more than £1bn, received the pay in shares, salary, and for other costs.

The BBC has contacted Apple for comment.

In a letter to shareholders, the ISS said there are "significant concerns" over the "design and magnitude" of the package. "Half of the award lacks performance criteria," ISS said.

After the email was sent, Apple share price regained ground by 2%.

Some analysts believe Apple’s stock is falling because it places inordinate faith in its smartphone the iPhone at a time when smartphone sales globally are beginning to peak.