US Edition The retailer had been one of the most immune from the high street slowdown this year, but the company said that sales had deteriorated markedly in the last few weeks.
However, it added that it was nonetheless on track to hit City profit forecasts.
In the summer the company announced that in the seven weeks to 17 September sales had increased of 3.3pc, but since then there had been a marked decline in consumer confidence. It warned a tough Christmas was coming.
The group, which operates 350 JD Sports and Size? outlets, said like-for-like sales in the 16 weeks to November 19 fell 1.5%, compared to 1.6% growth in seven weeks to September 17.
Nick Bubb, the veteran retail analyst, said: "The young kids who form JD’s customer base have money problems as well these days. So, it’s not too surprising that JD Sports has seen trading weaken quite sharply over the last couple of months, with ex-VAT LFL sales in its core JD chain down by 1.6pc in the last 16 weeks, implying around a 4pc decline in the last 9 weeks."
However, some analysts said they were quite relaxed about the trading update, saying it was no worse than expected, considering the conditions on the high street.
Freddie George at brokers Seymour Pierce said: "There is, we believe, significant potential to develop the company's own label ranges, grow its internet sales and develop the wholesaling activities.
"The company is also likely to get some help next year from the European football championships and the Olympics. This is one of our favoured stocks in the sector and the company has a relatively strong balance sheet with net cash forecast at c£100m at end of January 2012. We are keeping our Buy recommendation."
The shares fell 32.5 to 782.5p by mid morning.