UPDATE 1-ProSieben downbeat on 2008 ad market, shares slump

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FRANKFURT, April 25 (Reuters) - Shares in ProSiebenSat.1 (PSMG_p.DE: Quote, Profile, Research, Stock Buzz) tumbled by more than a quarter on Friday after Germany's largest commercial broadcaster posted a sharp drop in quarterly core profit and gave a bleak outlook for 2008.

The shares fell 26.6 percent to 9.97 euros by 1250 GMT, making the Munich-based company the biggest loser in all the German indexes. In the past year, the stock has lost 48.2 percent in value, according to Reuters data.

ProSieben, which bought SBS Broadcasting last year for 3.3 billion euros ($5.15 billion), posted a 25 percent drop in first-quarter recurring core profit of 88.5 million euros as sales dropped 2 percent to 729 million.

Core profit at the German-speaking TV business fell 18 percent to 57.6 million euros.

As a consequence, its head of marketing and sales, Peter Christmann, announced his resignation.

Chief Executive Guillaume de Posch, who told Reuters in January he "felt positive about 2008", said in a conference call the company was prepared for a difficult second quarter and cautious in its outlook for 2008.

"Revenue in the German-speaking region will be down in the second quarter," De Posch said, adding the German TV advertising market was expected to be flat or no more than one percent up.

"In light of the first quarter, we will not be able to match that (industry figure)," De Posch added, although he expected the second half of the year "to be better, given no recession hits the German market".