Customers who are thinking of buying the new iPhone 6s and iPhone 6s Plus and want to receive them at launch will have to brave the long lines outside Apple's retail stores on Friday, Sept. 25, when Apple begins to sell the new phones in its physical locations.

As first noted by 9to5Mac, Apple's online store has officially run out of stock of iPhone 6s and iPhone 6s Plus units for preorder in the United States and the 11 other countries where the new iPhones are launching at the end of the week.

Higher demand for the 5.5-inch iPhone 6s Plus more quickly exhausted Apple's preorder stocks last week, while the 4.7-inch iPhone 6s ran out on Monday, four days ahead of the iPhone's launch date. The earliest customers can expect to receive the iPhone 6s bought on the online store as far back as one to two weeks, while the iPhone 6s Plus has been pushed back by as much as four weeks.

The only way to get an iPhone 6 or iPhone 6s Plus on launch day is to line up outside one of Apple's retail stores and hope there are still some units left by the time you reach the counter. However, not all locations will sell to walk-in customers. Apple stores in China, Hong Kong, Japan and tax-free U.S. states will require a reservation.

The news is not surprising, since Apple has always been known to run out of stock on launch day. It is unclear if the shortage is due to extremely high demand or supply constraints as predicted earlier by KGI Securities analyst Ming-Chi Kuo. It is also possible that it is a combination of both.

Apple CEO Tim Cook earlier announced that the company was on pace to break its own record of selling 10 million iPhones last year. However, it is worth noting that last year's preorder period was one week shorter than this year's, which means Apple could have anticipated the bigger demand and stocked up sufficiently to fulfill all preorders.

This year's launch also includes China, which wasn't included as a launch country for the iPhone 6 and iPhone 6 Plus. Last year, Apple's China launch was reported to have reached 20 million units sold on the first weekend, apart from the 10 million units sold during Apple's earlier launch in the U.S.

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