Anticipation for the new model has led to a drop in sales for the current devices, causing Apple to miss analysts' sales and profit targets for the three months that ended in June. The iPhone accounted for 46 per cent of the company's revenue in the quarter.
Apple is battling Samsung Electronics for supremacy in the US$219.1 billion ($270.1 billion) smartphone market.
While Apple's strategy has been to release a single smartphone each year, Samsung has become the world's leading handset maker by putting out several devices a year in a range of sizes and prices.
The planned September debut was reported earlier by iMore, a technology news website. Natalie Harrison, a spokeswoman for Apple, declined to comment.
Signs that Apple is preparing to introduce a new product were evident in its quarterly financial results.
The company increased the prepayments it is making for components by US$1.15 billion - a jump that may indicate a new product is going to be introduced, according to a report from Maynard Um, an analyst at Wells Fargo Securities.
Apple co-founder Steve Jobs had worked closely on the redesigned phone before his death in October, a person familiar with the matter said.
- Bloomberg