Carl Icahn, the veteran activist investor, has taken a "large position" in Apple and is pushing for a further return of cash to shareholders, a move that threatens to reopen the debate about the iPhone maker's vast cash holdings just months after it saw off another challenge from agitating shareholders. Mr Icahn's postings on Twitter disclosing his stake and discussions with Apple's chief executive on Tuesday boosted Apple's valuation by more than $12bn. After trading around $476 immediately before the tweets, Apple closed up 4.75 per cent for the day at $489.57, its highest since January's disappointing earnings. Tim Cook, chief executive, resumed Apple's dividend payouts last year, something its late co-founder Steve Jobs had been averse to doing. "Had a nice conversation with Tim Cook today," he said on Twitter. Mr Icahn did not disclose the size of his stake but several reports suggested he bought $1bn worth of shares, pegging his Apple holding at around a quarter of one per cent. Mr Einhorn's last reported stake is worth just over $1bn at Apple's latest valuation. Apple's largest shareholder, the Vanguard Group, an American fund manager, holds 4.76 per cent of its stock, according to a March filing. Apple's shares have twice dipped below $400 this year and remain well below their highs of September last year, as investors become concerned about competition in the smartphone market and slower growth. Rallies in the stock in the aftermath of April's announcement of an increased payout had not been sustained, despite bringing an end to Mr Einhorn's campaign. It held just under $130bn in net cash, more than two-thirds of it outside the US. Some analysts believe Mr Icahn's presence could accelerate that moment. In a note to clients, S&P Capital IQ's Scott Kessler said: "We believe Icahn's position in, and potential role as an activist in, [Apple] could have a notable positive impact on the stock."