Last-minute pre-Christmas shopping resulted in the strongest weekly gain in comparable retail sales seen since July, according to a trade group.

Comparable sales – or sales at stores open at least a year, a key measure of retail performance – rose 4.3% year-over-year for the week ended December 25, according to an index of 75 major chains compiled by the International Council of Shopping Centers, a New York-based trade group.

That was the strongest week retailers have seen since July 3, when they recorded a weekly comparable-sales increase of 4.4%, according to the index. As a result, retail chains are likely to meet lowered expectations for the season, said the chief economist and director of research at the trade group, Michael Niemira.

“Although holiday sales were slow and uneven for most of the season, last minute shoppers were out in force last week,” Mr. Niemira said.

Strong categories included winter clothing, electronics, and jewelry, the trade group said. However, Credit Suisse First Boston said in a research note yesterday that, according to its own channel checks, outerwear was marked down at 50% off in some cases, and sweaters were being promoted more broadly and deeply than last year.

“We believe this is an indication that, although sales may have been strong, gross margins will be pressured at many retailers,” the firm said.

Mr. Niemira said he expects December’s comparable sales to rise 3% to 3.5%, and backed his earlier forecast for sales during November and December combined to increase 2.5% to 3%, versus a 4% increase last year. That holiday forecast had been lowered from an initial estimate for a 3% to 4% gain after retailers reported a disappointing increase of 1.8% in November.

On a week-over-week basis, comparable sales last week increased 2.7%, after recording somewhat smaller week-over-week gains during the second and third weeks of December. That is an indication that shoppers increasingly are procrastinating on their holiday purchases with each new season.

But if that leaves more room for upside for holiday sales, it also creates more uncertainty about what the final numbers will be, say two analysts at Piper Jaffray Company in Minneapolis, Jeffrey Klinefelter and Neely Tamminga. They note that, in addition to last minute shopping, consumers increasingly are turning to gift cards, which can push the recognition of holiday revenue farther out still.

“Sales of gift cards aren’t recognized until they are redeemed,” the analysts said yesterday in a research note.

An index compiled by Piper Jaffray currently expects comparable sales in December to increase 2%,versus a 3.9% gain last year. The firm predicts that mass merchants like Wal-Mart Stores and Target will post a 2% gain, versus a 4% increase last year. Specialty apparel chains such as Gap, and teen and women’s apparel chains like Urban Outfitters and Ann Taylor Stores collectively will post slightly more modest comparable-sales increases, the firm predicts.

While markdowns have been more plentiful this year amid high gasoline prices and a stalled stock market, “the industry spent the year getting inventories right and tight and we believe, in the absence of this discipline, many retailers could have faced a much bleaker December,” Piper Jaffray said.

In anticipation of December retail sales reports, which are slated for release next week, other industry surveys generally have predicted solid, if not spectacular, results. The National Retail Federation, a Washington-based trade group, expects holiday sales to be up 4.5%, versus a 5.1% increase last year.

Over the weekend, MasterCard Advisors said holiday spending rose 8.1%, although that figure incorporates additional online sales and sales of gift cards, as well as the increased use of credit over cash.

The consulting unit of MasterCard International said sales were driven by higher-income consumers, and that sales of apparel and home furnishings outpaced sales of books, music and videos.