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.

In an impassioned plea, Cook, boss of the world's largest company, says that the epidemic of false reports "is a big problem in a lot of the world" and necessitates a crackdown by the authorities and technology firms.

He calls for a campaign similar to those that changed attitudes on the environment to educate the public on the threat posed by fabricated online stories. Made-up news reports trying to promote a particular agenda gained huge traction on social media in the US during the election.

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.