SEATTLE (REUTERS) - Mr Jeff Raikes, the chief executive of Bill Gates' US$38 billion (S$48 billion) philanthropy organisation announced his retirement on Tuesday, without a successor being named.

Mr Raikes, a former Microsoft Corp executive, has been chief executive of the Seattle-based Bill & Melinda Gates Foundation since 2008.

The Nebraska native has been considered by some observers as an outside candidate to be Microsoft's next chief executive, but he gave no indication that might be in his plans.

"I'm looking forward to doing some things I haven't had time for, including my work at the Raikes Foundation, which is tackling youth and education issues," he said in an email to employees at the Gates Foundation.

Mr Raikes will remain in his position until a successor is named, the foundation said.