Carl Icahn has raised his stake in Apple by 22 per cent and heaped pressure on the company's board to hand back more cash to shareholders.
The US billionaire said he now owned 4.7 million shares in Apple, worth about $2.5bn (£1.5bn). However, he believes they are undervalued and wants the company to buy back $150bn of shares from investors to boost their price, rather than sitting on its enormous cash mountain.
In a letter to Apple's chief executive Tim Cook, published on his new website, Mr Icahn said: "We want to be very clear that we could not be more supportive of you, the existing management team, the culture at Apple and the innovative spirit it engenders.
"Our criticism relates to one thing only: the size and timeframe of Apple's buyback program. It is obvious to us that it should be much bigger and immediate."
Mr Icahn reckons Apple's shares could hit $1,250 under his plan. They were at less than half that yesterday.
His decision to go public was criticised by Bill Gross, co-founder of the investment manager Pimco. He wrote on Twitter: "Icahn should leave Apple alone & spend more time like Bill Gates [in philanthropy]. If Icahn's so smart, use it to help people not yourself."
Mr Icahn said: "While the board's actions to date may seem like a large buyback (a $60bn share repurchase over three years), it is simply not large enough given that Apple currently holds $147bn of cash on its balance sheet."