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(Pocket-lint) - Apple's iPhone 5S and iPhone 5C are set to release tomorrow, though it appears O2 in the UK won't have any iPhone 5S smartphones to dole out in stores on launch day (update below).

The company tweeted on Thursday - just one day before the iPhone 5S is set to release - that UK customers can buy the iPhone 5C in stores but must order the iPhone 5S online or on the phone. It also added the following: "No 5S in stores but they'll order for you and say when it's in."

One Twitter user asked O2 if that meant no O2 stores would have the iPhone 5S in stock. The company replied with a "yes" and explained that the smartphone is only available online or through call centres starting at 8am on Friday. O2 also mentioned that its Refresh program is exclusive to O2, so customers would get a standard contract if they decide to buy the iPhone 5S direct from Apple.

O2 has therefore confirmed it doesn't have any iPhone 5S in stock at stores, which would align with previous reports about highly constrained iPhone 5S inventory. Pocket-lint has reached out to O2 for clarification on the matter, and we'll update when more information is available (update below).

Read: iPhone 5S supply is highly constrained - better get in line early

Multiple reports - citing unnamed sources - claimed earlier this week that iPhone 5S supply is “severely constrained” and "grotesquely unavailable". The extremely low stock is allegedly the reason why Apple couldn't partner with carriers and retailers to offer pre-orders.

Simply put: Apple had to ensure there would be enough iPhone 5S units to sell in Apple stores on 20 September. However, by its doing so, companies like O2 in the UK don't have any in-store devices to sell come launch day on Friday.

Update 1: O2 has posted the following message on its iPhone 5S page (as seen above): "Order your iPhone 5S in store and we’ll get it to you as soon as we can - you won’t be able to collect on launch day." 

Update 2: O2 has issued the following statement to Pocket-lint: "We won’t have iPhone 5S stock in stores to sell but customers will be able to place their order with us in store. They can also order online or over the phone. If customers are in store they’ll be able to get a demo of the phone and place their order with the help of one of our store advisers. We anticipate that orders made in our stores for the 5S will be available to collect at the store in three to six weeks. The customer will receive a text when their handset is ready for collection." 

Writing by Elyse Betters.