Bill Gates is mulling a return to Microsoft according to a well-placed source. The mole told Fortune that the tycoon may consider clasping the reins of the company he co-founded and replacing Steve Ballmer as CEO.

The remarks came from "a prominent chief executive", according to the site. Gates left Microsoft to focus on philanthropy in June 2008, though he is still the chairman of the board and the company's largest shareholder.

Right now, Gates is focused on nuclear power plants in China, but there are a couple of factors that could send him back to Redmond desk.

One would be the Microsoft stock value. Earlier this year, activist investor David Einhorn of Greenlight Capital likened Ballmer to a weight dragging down the company's share price. He said Ballmer was "stuck in the past" and called for him to step aside. Ballmer is not loved by Microsoft's less vocal investors either.

Another spur for Gates could be the desire to give Microsoft a new focus in a time of sharp change, especially as it pushes into the mobile space. Steve Jobs and Larry Page famously returned as CEOs after absences and pulled off marvellous recoveries. But despite Jobs' claim that Gates always copies him, it's not likely Gates will do something just because Steve did.

Microsoft told us that they do not comment on rumour and speculation. ®