BHP Billiton Group CEO Andrew Mackenzie is facing uncertainty over his tenure in the group as the company's annual general meeting approaches and as Chairman Ken MacKenzie looks to further shuffle the board, The Daily Telegraph reported Oct. 7.

According to the report, investors called for the company to start succession planning as Mackenzie is set to mark his fifth anniversary at the helm of the company in March 2018. Mackenzie has been under pressure to improve shareholder returns, and recent reports said he has been given six months to come up with a plan to turn the company around.

"The number-one item that comes up in conversation between investors is Mackenzie," one shareholder said, while another was cited as saying, "We think the culture within BHP needs a good shake, if not a change," suggesting that the company would need to look at external candidates.

BHP Chairman Ken MacKenzie is understood to be encouraging board members to consider stepping down after nine years, which could see three directors retiring in the next 12 months.

The mining giant declined to comment on the matter but is set to hold its annual general meeting in London on Oct. 19 and in Sydney on Nov. 16.