Two thirds of customers buying Apple’s iPhone are switching from rival networks, O2, Apple’s partner, said yesterday, as it sought to counter evidence that sales had failed to meet expectations.

The mobile giant, Apple’s UK network partner, said that the device was its “fastest-selling”, with “tens of thousands” sold since its launch on Friday.

Two thirds of users, O2 said, were customers who had been lured away from rival networks such as Vodafone, Orange and T-Mobile. In the long term, the operator expected the figure to rise to three quarters of the total.

The group’s boast came amid anecdotal reports that sales of the device — a combined iPod music player, mobile phone and internet browser — were slower than expected at stores in areas including Newcastle, Manchester and Birmingham.

The refusal of Apple, O2 and Carphone Warehouse — the only independent retailer to sell the device — to release detailed sales figures further fuelled speculation that they were not living up to expectations.

However, the claim that two thirds of users are being drawn from rival networks will further alarm the mobile operators that lost out on the deal. They are preparing to fight back over the key pre-Christmas sales period to stop high-spending subscribers switching to O2 to secure an iPhone.

Arun Sarin, Vodafone’s chief executive, is expected to be quizzed on the deal’s likely impact on its figures at the company’s results presentation today.

The British mobile market is so saturated that there are more mobiles than people and yesterday O2’s own results revealed the increasingly difficult trading conditions. O2’s third-quarter figures to September 30 revealed that its pre-pay customer base fell by 44,000. Peter Erskine, O2’s chief executive, said that he expected this lower-end market to decline in the UK for the first time this year.

Although the decline is partly because of networks’ efforts to move users on to contracts, it also reflects Britain’s high mobile penetration.

The desperation of the main networks — Vodafone, Orange, T-Mobile and O2 — to gain an edge over each other triggered a fierce battle for the rights to market the iPhone. The losing players maintain that the O2 deal is not economic. The Spanish-owned group is thought to have agreed to an ongoing share of revenues with the Californian giant of up to 30 per cent.

Yesterday rival operators sought to dismiss the prospect of their own trading being significantly affected by customers switching to O2. They said that losing customers initially was no indicator of the long-term impact of the deal. The early iPhone subscribers represented a hardcore of Apple fans who were willing to pay to terminate their existing contracts and switch to O2.

The operators also suggested that the high price of the gadget meant that it would appeal only to a niche, high-spending audience.

The basic iPhone handset costs £269 — £69 more than in the United States. A contract with O2 costs between £35 and £55 a month for a minimum of 18 months.

However, after losing out on the deal, many rival operators have rushed to implement their own rival music deals. Vodafone, for example, has a tie-up with MusicStation, a service that offers its subscribers access to a vast catalogue of music for less than £2 a week.

Microsoft has signed an exclusivity agreement with Musiwave that could lead to a acquisition of the French mobile phone music company, which offers services including ringtones, music downloads and video clips for mobile phones. No timetable nor any financial terms for the possible deal have been disclosed.

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