General Motors' veteran product development expert Bob Lutz is to retire at the age of 78, a surprise departure that continues the upheaval at the Detroit giant's top level.

Lutz, vice-chairman of GM, has been an unofficial eminence grise through theturbulent path through bankruptcy and downsizing. He goes at the beginning of May, reversing repeated recent declarations he had no intention of leaving.

"My work is done here," Lutz wrote in an email to the Associated Press from Geneva, where he is attending the Swiss city's motor show. "The whole organisation, top to bottom, now has absolute product superiority as the highest objective which enables all others. So I can retire in peace."

GM's brands include Chevrolet, Cadillac, Vauxhall, Opel and Buick. Long regarded as America's top carmaker, GM was overtaken by Ford in sales last month for the first time since 1998.

Lutz has been a plain-speaking figure throughout his 47 years in the motor industry. He has bucked GM's official line by expressing scepticism about the commercial prospects for electric or hybrid vehicles. In January, he predicted the internal combustion engine would reign supreme for at least a further decade, and that it would be "years and years" before alternatives make up even a tenth of the market.

In 2008, he described global warming as "a total crock of shit", although more recently he has helped develop the Chevrolet Volt, a potentially ground-breaking hybrid with a generator on board, and even showcased it on David Letterman's US chatshow.

GM has had substantial turnoverat the top since emerging from chapter 11 bankruptcy in July. Chairman Edward Whitacre, a former telecoms boss hand-picked by the Obama administration, took control in December, ousting chief executive Fritz Henderson after hehad been in the job a mere five months.

Whitacre paid tribute: "The influence Bob Lutz has had on GM's commitment to design, build and sell the world's best vehicles will last for years to come."