Speculation about a 12 September product unveiling has boosted Apple to become the most valuable public company in history, taking the title from Microsoft, which has held the record since December 1999.

On Monday, Apple’s stock reached $665.15 (£422), and the company’s most recent figures list 937.41 million shares outstanding, giving Apple a market capitalisation of $623.5 billion (around £400bn). 

That figure beats Microsoft’s record of $618.89 billion, which earned the tech giant the title of most valuable public company on December 30 1999, when Microsoft’s shares peaked at $119.94.

AllThingsD notes that, if taking inflation into account, Microsoft would still hold the title of the most valuable public company by a wide margin, with an approximate value of $850 billion in 1999. At present, Microsoft is valued at around $257.6 billion.

Apple’s shares have risen 64 per cent in 2012, and its record market cap has led it to be worth 53 per cent more than oil firm Exxon Mobil Corp, the second most valuable public company, which Apple passed last year with a value of $347 billion. Also in the top five most valuable companies are Wal-Mart and IBM.

The rocket in Apple share price is likely to have been caused by talk of the company’s predicted 12 September special event, which is expected to see the biggest consumer electronics launch in history. It is believed that Apple will launch the iPhone 5 at the event, along with an iPad mini, and possibly an Apple television or set-top-box.

Meanwhile, Facebook’s shares dipped to a record low of $18.75 on Monday, less than the $38 they were sold for in May.