For the first time in more than a year analysts are cutting their forecasts for Standard & Poor’s 500 Index earnings, jeopardizing gains from the largest September rally since World War II.

Bloomberg reports that estimates for S&P 500 companies’ combined 2011 profit fell as low as $95.17 last month from an August high of $96.16 and posted the first quarterly reduction since the three months ended June 2009, according to more than 8,500 analysts the news service tracked. The revision came as the benchmark for U.S. equities rose 8.8% last month, the largest September advance since 1939.

Now, money managers at Stifel Nicolaus & Co. and USAA Investment Management Co. are preparing for weaker returns in October as Alcoa Inc.’s Oct. 7 report starts the third-quarter earnings season. But Bloomberg notes that even with the decline in analyst estimates, bullish analysts say equities remain cheaper based on forecast profits than at any time since 1988, excluding the six months after Lehman Brothers Holdings Inc.’s bankruptcy in 2008.

“Earnings forecasts are going to be somewhat more muted in their expectations for future growth,” Chad Morganlander, a Florham Park, N.J.-based money manager at Stifel Nicolaus, told Bloomberg. “It’s not necessarily a bad thing, but it’s certainly not the fuel that will reignite animal spirits.”