ZURICH, Aug 4 (Reuters) - Shares in Swiss auto parts and textile group Rieter (RIEN.S) tumbled on Tuesday 4 August after it announced the departure of its chief executive and said first-half sales had slumped due to the economic downturn.

The group, which also gave a gloomy outlook, said it would post a some 50-percent drop in first-half sales and a net loss of 150 million Swiss francs ($141.4 million). Rieter will post final half-year figures on Aug. 12.

"The Board of Directors expects the market environment to remain difficult for the next two years. The extensive restructuring measures initiated in 2008 call for further vigorous efforts," Rieter said in a statement.

By 0740 GMT, shares in the group had fallen 6.3 percent to 192.00 Swiss francs, underperforming a near flat Swiss mid-cap index.

"Rieter is heading towards a scenario, described as 'worst case' at the 2008 press conference end-March 2009," Kepler Capital Markets analyst Fabian Baumann said in a note.

Rieter, which has already said it will post a full-year operating loss in 2009, is grappling with the collapse of both the auto and textile industries, which has snuffed out demand for its products.

But the group said it was confident of reaching a turnaround in 2010 and of reporting positive operating results in 2011.

Rieter said the board of directors wanted to play a more active role in the running of the group and this was behind the departure of CEO Hartmut Reuter, who has held the position since 2002.

Chairman Erwin Stoller has been elected as executive chairman and the heads of Rieter's textile systems and automotive systems, as well as the head of the corporate centre, will report directly to him. ($1=1.061 Swiss Franc) (Reporting by Katie Reid; editing by Simon Jessop)