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(Pocket-lint) - The iPhone 5S and iPhone 5C will release on Friday - but good luck getting your hands on the iPhone 5S come launch day. Stock  is "severely" and "grotesquely" low, according to reports.

BGR reported - citing an unnamed source - that Apple's iPhone 5S supply is “severely constrained”. This is allegedly the reason why Apple couldn't partner with carriers and retailers for pre-orders. Simply put: the company had to ensure there would be enough iPhone 5S units to sell in Apple stores on 20 September.

Following BGR's report, AllThingsD claimed that sources at multiple US carriers described iPhone 5S inventory as "grotesquely unavailable". Although the sources complained of "extremely disappointing shipments" from Apple, they also speculated that demand will be lower than expected because the iPhone 5C is currently wooing many customers.

Citi analyst Glen Yeung forecasted that Apple sold 2.2 million pre-orders for the iPhone 5C within the first 24 hours of availability last Friday, according to CNET. In comparison, Apple's iPhone 5 only sold 2 million pre-orders within the first day. Yeung's estimation mirrors earlier claims from Phones 4u.

Phones 4U said it experienced "phenomenal" demand in first few days. Specifically, as reported by Telecompaper, the UK mobile phone retailer saw "double digit growth in pre-registrations compared to the same period for iPhone 5 in 2012, and expects the new handsets to record the most pre-registrations ever at Phones 4u".

Read: More than 2 million iPhone 5 pre-orders made in first 24 hours

Going back to the constrained iPhone 5S, 9to5Mac has reported that Apple is expecting the space grey model to be most popular. It's even making sure three out of every four units will be a space grey model in Apple stores, whereas the other options - such as the gold model - will be hard to find on launch day.

Have no fear, though. Apple is purportedly working on an update for its Apple Store app that will allow customers to see when new iPhone models are available at their local Apple Store. The update might hit sometime next week.

Writing by Elyse Betters. Originally published on 17 September 2013.