O2 has confirmed that iPhone 5s and iPhone 5c buyers will be unable to use its 4G network at launch.
The mobile operator said the situation, which it has blamed on a settings issue, will be rectified "within the coming weeks".
Until the update, customers can still use all the data that comes with the 4G tariff.
Both devices will be available to pre-order from 13 September, and will be released in the UK on 20 September, Apple has confirmed.
In the meantime, customers who purchase either model on an O2 4G tariff will receive a 5 discount off their bill for that month.
"Until the update, customers can still use all the data that comes with the 4G tariff," a company statement read.
The announcement goes some way to explaining why O2 was left off Apple's list of network operators that will support 4G LTE on the iPhone 5s and iPhone 5c, while Vodafone and EE were both included.
O2 switched on its 4G network on 29 August, which was the same day as arch-rival Vodafone.
The next-generation iPhones were unleashed on the world earlier this week, with the Cupertino tech giant making a play for the budget end of the smartphone market with the iPhone 5c.
However, with a SIM-free starting price of 469, many industry watchers have declared the devices too expensive, including IT Pro technical editor, Khidr Suleman.
O2 has confirmed its contract pricing for both devices. Both will be offered to customers on its Refresh tariff, which leaves users free to upgrade to a newer device without incurring fees to break the remainder of their contract.
It also means existing O2 Refresh customers can upgrade to the iPhone 5s or iPhone 5c, without having to wait for their existing contract to run out.
The 16GB iPhone 5s will set users back 119.99 upfront, and 37 a month for unlimited texts and minutes, as well as 1 GB of data.
Customers that plump for the 4G option will have to fork out the same upfront cost, and will get unlimited texts, minutes, and 5GB of 4G data for 47 a month.
For a 49.99 upfront cost, O2 is offering users a 16GB iPhone 5c for 32 a month with unlimited texts, 600 minutes and 750 GB of 3G data.
Alternatively, they can get the same device for 99, with a 42 a month cost for unlimited texts, minutes and 5GB 4G data.
