By Chris Bryant in Frankfurt

Daimler has replaced the head of its Mercedes-Benz operations in the US without explanation, casting a cloud over one of the company?s most important markets.

A Daimler spokesman confirmed on Tuesday that Ernst Lieb, 56, – a more than 30-year Daimler veteran who oversaw strong sales growth of its luxury vehicles in the US this year – had been ?relieved of his duties? as chief executive of Mercedes-Benz US, with immediate effect.

The Stuttgart-based maker of luxury vehicles and trucks said Herbert Werner, chief financial officer at MBUSA, would assume the top job until further notice. Mr Lieb has not left the company, a development that is not thought unusual given Germany?s strict labour laws.

Although Daimler gave no further explanation, it is thought the decision is unrelated to operational performance or accounting issues.

Analysts and investors were, however, left frustrated by the lack of any further information from Daimler. ?Shareholders definitely expect an explanation given the US is Mercedes? biggest export market,? said Arndt Ellinghorst, head of automotive research at Credit Suisse.

Mr Lieb, who took over the role in 2006, has spurred solid growth this year in the ultra-competitive US market. Mercedes-Benz has sold 170,058 vehicles there since the start of the year, an increase of 6.5 per cent on the same period a year ago. Its rival BMW has increased US sales by 13 per cent to 177,679 units so far this year.

Last year Mercedes-Benz sold 220,500 vehicles in the US, more than the 160,000 it sold in China and not far short of the 293,000 figure for Germany. In July the company announced it would invest more than $2bn in its manufacturing plant in Alabama, where it builds several of its sports utility models.

In the wake of Mr Lieb?s departure, Mercedes abruptly cancelled a national meeting of its US dealers, on Tuesday and Wednesday in Chicago.

During his five years as Mercedes? US chief, Mr Lieb significantly improved the company?s relations with its dealers and was popular among them.

He was instrumental in persuading dealers to invest more than $1bn since 2008 in upgrading their facilities to help improve the brand?s image. The company in May unveiled a 330,000 sq ft flagship showroom in Manhattan.

Mercedes-Benz also announced last week that it had bought the rights to lend its corporate name to the Louisiana Superdome stadium in New Orleans, which once sheltered thousands of displaced victims of hurricane Katrina.

Daimler?s shares fell 1.7 per cent to 35.89 euros, nderperforming the broader blue-chip Dax index.

Additional reporting by Bernard Simon in Toronto and John Reed in London

? The Financial Times Limited 2011