FRANKFURT, Sept 11 (Reuters) - Otto Beisheim Group said it was quitting an investor pact in German retailer Metro (MEOG.DE) to be freer about what to do with its 18.5 percent stake but would remain a major shareholder.

Concern that Beisheim might sell his stake weighed on Metro shares which were down 2.3 percent at 36.02 euros by 1030 GMT, with the German blue-chip DAX index .GDAXI up 0.6 percent.

Otto Beisheim Group and the Haniel and Schmidt-Ruthenbeck families form a shareholder pool together controlling about 66 percent of the world's fourth largest retailer. The group said it would move its stake out of the pool as of Oct. 1.

"Otto Beisheim will continue to be a committed founding member and one of the major shareholders in Metro, even after exiting the pool," Beisheim said in a statement.

"The market is speculating about Haniel possibly placing some of its stake in Metro, or that Beisheim might do so separately. The details are a bit shady, but some investors might believe that after Beisheim's pullout, the group would be forced to sell to cover the loss in investments," a trader said.

Two sources close to Metro told Reuters neither Haniel nor the Schmidt-Ruthenbeck family planned to reduce their stakes.

Haniel and Metro declined to comment.

(Additional reporting Tyler Sitte; Editing by Dan Lalor) (Reporting by Eva Kuehnen and Nikola Rotscheroth in Duesseldorf)