The release iPhone 6 and iPhone 5S is rumoured to be postponed due to US carrier T-Mobile's delayed availability of the current iPhone.

It has been less than a week since T-Mobile announced the long-awaited release of the iPhone 5 on its network free of contract. The smartphone will make its debut with T-Mobile on April 12. Pre-orders for the handset began for the iPhone 5 on Friday.

With the delayed availability of the handset in T-Mobile, Apple may not release the successors of the handset yet. Rumours claimed that Apple is set to unleash iPhone 6 and iPhone 5S in June.

T-Mobile no longer offers handsets on contract subsidies, as part of its reinvention as the "UNcarrier." Unlike the Big Three of AT&T, Sprint and Verizon Wireless, T-Mobile now allows buyers to purchase a smartphone at an inexpensive upfront cost, with the device to get paid off in installments over a span of 20 to 24 months.

T-Mobile's most affordable UNCarrier plan for the iPhone 5 would involve paying $99 upfront for the device in 16 GB form, and $20 a month over the next two years or until the phone is completely paid off, whichever comes first. The 32 GB version of the iPhone 5 would require a $200 down payment, while the 64 GB iPhone 5 will cost $300 upfront. For both higher-end versions, buyers will also have to pay $20 monthly for two years. Aside from the $20 minimum monthly payment for the iPhone 5, T-Mobile will, of course, require subscribers to pay for their monthly price plans.

According to Japanese magazine Mac Fan, the new iPhone 5S will be unveiled on June 20 and go on sale by July. It also said the rumoured low-cost version, targeted at India and China, would be launched in August. Mac Fan did not share where it got its news from. However, the new iPhone 5S will not be imbued with Apple's new mobile operating system, iOS7, it would seem.

With these new developments, it is most likely that Apple will release the next iPhones later this year.

Apple is expected to include an improved camera in the iPhone 6 and/or iPhone 5S which is rumoured to be capable of taking "Super HD" pictures. In order to take snaps at that resolution it'll need to have at least a 13 mega-pixel camera.

The lack of NFC support on the iPhone 5 was one of the biggest criticisms aimed at Apple's smartphone. Rumours suggest that the iPhone 5S could include support for the contact-less technology which can be used to pay for items in retail stores. Other features rumoured to be included in the handset is the A7 quad core chip processor, Retina or IGZO screen display among others.