American retailers including Wal-Mart Stores Inc. and Target Corp., the two largest discount chains, are facing slower-than-expected sales gains during the back-to-school season as rising gasoline prices erode shoppers’ budgets.

The International Council of Shopping Centers, which represents mall owners, reduced its estimate Tuesday for August after Wal-Mart cut its forecast for sales at stores open at least a year. Sales growth is probably slowing to as little as 2.5%, down from as much as 4% at the start of the month, said the chief economist for the group, Michael Niemira.

Discounters such as Wal-Mart are more at risk because lower-income shoppers tend to curb spending the most as the price of gas rises, he said. Sales began slowing during the past two months with gas prices 26% higher in July and retailers facing tougher comparisons to the same period a year earlier, when consumers were flush with money from tax credits.

“We’re likely to see a continuation of this softer pace,” said Mr. Niemira, who is based in New York and has been following retailing since 1984. “The expectation was that back-to-school wouldn’t be particularly strong, and not particularly weak.”

The August through September back-to-school season, when merchants get about one-sixth of revenue, is the second-biggest selling period after Thanksgiving to Christmas. A slowdown in consumer spending, which accounts for about two-thirds of American economic activity, may signal a decline in economic growth.

Wal-Mart, the world’s biggest retailer, said in a recorded message Monday that August sales will be little changed or rise as much as 2%, which would be the smallest gain in 17 months. Target also said in a message it expects results to be unchanged or to increase as much as 2%.

Mr. Niemira, who tracks 76 retail chains in his monthly survey, said Wal-Mart’s sales decline 0.77 percentage points for every 10% rise in gas prices. That compares with 0.36 percentage points for the industry as a whole.

Ericka Longley, 32, walked out of a Wal-Mart in Greensboro, N.C., on Monday without jeans and other school clothes she hoped to buy for her daughter, Destinie, who’s starting kindergarten.

“Prices for children’s clothing are outrageous and gas prices are ridiculous,” said Ms. Longley, a tissue-paper factory worker. She expects to spend 25% less on clothing for Destinie this year in part because of higher gas prices.

“I couldn’t spend as much as I wanted to” on back-to-school shopping at Wal-Mart, said Tina Moye, 39, a housewife in Stone Mountain, Ga., who pays $40 more a month to fuel her Chrysler Sebring. “I had to buy a little bit at a time since the gas prices were so high.”

Consumer spending slowed to a 1% annual rate in the second quarter, the weakest pace since the 2001 recession. The slowdown from April through June restrained the economy, which grew at a 3% rate, the weakest in more than a year.

Economists at Lehman Brothers Inc. and Citigroup Global Markets Inc. are among those who lowered economic growth forecasts within the last week because of higher fuel prices. Lehman Brothers cut its second-half growth estimate to 3.8% from 4.5%.

“Recent energy costs have pointed to a building drag on real GDP growth of close to three-quarters of a percentage point already,” said a senior economist at Citigroup, Steven Wieting. If current crude oil prices near $50 a barrel persist, the effect on the economy may be even larger.

The economy will grow at a 3.4% annual rate this quarter instead of the 4.5% in the previous forecast, Mr. Wieting said in a report. Over the last five years Wal-Mart’s August same-store gains averaged 6.4% compared with about 3.1% for Target.

Other chains such as American Eagle Outfitters Inc. that are less dependent on shoppers who stick to budgets are doing better. Denim sales were so strong at the teen clothing retailer that it cut a back-to-school promotion on jeans to one week from three, executives said earlier this month.

Laura Kucera, 45, said she planned to visit American Eagle during a recent trip to the Woodfield Mall in the Chicago suburb of Schaumburg, Illinois. Ms. Kucera said she’ll spend about $500 on both of her daughters – one a college student and the other a seventh grader.

“I don’t set a budget,” said Ms. Kucera, a resident of Dyer, Ind. “We just sort of buy as they get the school lists and find out what they need. Obviously, Target is more convenient, but this is a fun destination thing for us to do.”

Families with school-age children are expected to spend about $483 on school-related items on average, a gain of about 7.2% from expectations a year ago, according to the National Retail Federation.

Sales at J.C. Penney Co. in the first two weeks of August were better than expected, helped by purchases of vintage denim and girls’ ponchos, Vanessa Castagna, chief executive of department stores, catalog, and Internet, said in an interview.

Larger retailers including Wal-Mart, Sears, Roebuck & Co., and J.C. Penney have also added more uniforms.

The percentage of parents shopping for uniforms has doubled in the past 10 years to 21%, according to market research firm America’s Research Group, as more schools instituted dress codes.

J.C. Penney, which surveyed about 4,200 schools about policies, created an Internet site to promote uniform apparel. Sears has increased its selection by about 30% in the past two years, spokeswoman Lee Antonio said.

“Uniforms became a very attractive, easy-to-get-into business,” said analyst Marshal Cohen at NPD Group Inc., a market research company in Port Washington, New York. “If I can get your uniform purchase, I can get a lot of your other purchases.”