Carl Icahn is not known as a passive investor. One of the original brash corporate raiders of the 1980s, Icahn's trademark move is to quietly build a stake in a public company which he then uses to influence the management strategy.

Already under pressure on a series of fronts, Apple chief executive Tim Cook cannot have been best pleased on Tuesday when it emerged that the hedge-fund manager had built up a "large" stake in the iPhone maker.

Computer industry veteran Cook was always going to have a tough job filling the shoes of Apple's maverick founder, Steve Jobs, but his headaches seem to be multiplying on an almost daily basis.

Earlier this week Apple shares were down almost 30 per cent from a high of $705.05 achieved in September last year. The Californian company known for its market-making innovations hasn't responded to advances by internet giant Google on the software front and Korean smartphone maker Samsung on the hardware side.

All eyes were turning to the introduction of the next generation of Apple’s cash cows – the iPhone and iPad. The sixth iteration of the iPhone is expected to be unveiled on September 10th, with a slimmer iPad being introduced in time for Christmas.. The success of both devices is seen as crucial to the future of Cook and the company.

Just to add further pressure, Oracle chief executive Larry Ellison said Jobs, a friend of his, was "irreplaceable" and Apple would struggle to stay ahead of the competition without him.

So Cook could have been forgiven for gulping when Icahn called Apple’s Californian HQ to announce he was now a significant shareholder. However, both sides reported they had a positive conversation – not something chief executives who discover the billionaire has joined their share register usually report.

For once Icahn may prefer to be a passive investor for now – he is involved in increasingly bitter and very public spats with management and other shareholders at diet supplement maker Herbalife and computer manufacturer Dell.

Icahn has been clear about why he is interested in Apple. With almost $150 billion (€113 billion) in cash on its balance sheet, it has committed to completing a $100 billion share buyback by the end of 2015. He believes this will push the share price above the $700 mark and it could prove to be a self-fulfilling prophecy.

Reports of Icahn’s involvement pushed Apple stock up 5 per cent at one point yesterday.

With Icahn on board Cook can expect to come under even more pressure to share some of those billions – a significant proportion of which is held offshore in Irish subsidiaries – with his shareholders.