DETROIT (Reuters) – General Motors Co posted its biggest quarterly profit in six years yesterday and CEO Ed Whitacre stepped aside on the cusp of an IPO (initial public offering) expected to allow the US government to relinquish its majority stake.

Whitacre, who has served just eight months as chief executive of the top US automaker, said he would step down on September 1, to be replaced by Dan Akerson, a GM board member and a managing director at private equity firm The Carlyle Group.

Whitacre's departure had been expected but the timing of the announcement was a surprise and came just a day ahead of GM's expected filing for a landmark stock offering.

Whitacre, who continued to commute from his home in Texas during his stint as CEO of the Detroit-based company, had said repeatedly that he would be an interim leader at GM.

"It was obvious that I was not going to be at GM for the long haul," Whitacre said in a surprising announcement at the end of a conference call to discuss the company's second-quarter earnings. "We have put a strong foundation in place, so I am very comfortable with my timing."

Akerson, a former CEO at Nextel, will become GM's fourth chief executive in just a year and a half, underscoring a continued challenge for an automaker analysts see as still in the early stages of a turnaround.

Separately, GM posted a second-quarter profit of $1.3 billion in evidence of a turnaround driven by cost-cutting in its 2009 bankruptcy and better sales in the United States.

The second-quarter profit was the largest since 2004, when the US auto market was still booming with annual sales of near 17 million vehicles and GM's brands accounted for more than one in four purchases of new cars and trucks.

The results reflected a 47 percent snap back in global production from the depressed levels of a year earlier when GM began operating under bankruptcy protection in a restructuring that included $50 billion in U.S. government funding.

Revenue rose to $33.2 billion from $31.5 billion in the first quarter, boosted by higher sales of more profitable new models such as the Chevrolet Equinox.

Analysts expect the company to use the results to build the case for a record stock offering and allow the U.S. government to reduce its 61 percent ownership stake.

A successful GM IPO would provide the Obama administration with evidence that the unprecedented and unpopular intervention in the U.S. auto industry has been a financial success.