Bill Ackman said his Pershing Square Capital Management LP hedge fund isn’t looking to sell its stake in General Growth Properties and urged the mall owner to enter negotiations for a takeover by Simon Property Group.

General Growth should “immediately form a special committee” and “initiate negotiations with Simon promptly,” Ackman said in a letter filed yesterday with the Securities and Exchange Commission. Ackman, whose company is General Growth’s second-largest shareholder, first said on Aug. 23 that Simon was interested in buying the Chicago-based company.

General Growth, the No. 2 U.S. shopping-mall owner, exited bankruptcy protection in November 2010 following a takeover battle between Indianapolis-based Simon, its larger competitor, and an investor group that included Pershing Square and Brookfield Asset Management.

General Growth filed for bankruptcy in 2009 after weighing itself down with $27 billion in debt that it was unable to refinance because of the financial crisis and collapse of the commercial mortgage-backed securities market.

David Keating, a spokesman for General Growth, declined to comment on the letter.