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O2 sought to dismiss reports that the Apple iPhone is selling at a slower than expected pace claiming the device was its "fastest-selling” ever.

Peter Erskine, the chief executive of the mobile group, which is Apple’s chosen network partner in the UK, said: “It has been the fastest-selling device we have ever seen.”

The phone had sold “in the tens of thousands” since its launch on Friday, he said.

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Britain’s biggest mobile operator also raised speculation about the potential hit to rivals such as Vodafone and Orange from its exclusive deal with Apple.

Two thirds of customers buying the iPhone were new customers to O2, Mr Erskine said, although he refused to detail from which networks they had come.

IPhone buyers must sign up to an O2 contract costing from £35 to £55 a month for a minimum of 18 months.

However the refusal of both O2 and Carphone, the only independent retailer to sell the phone, to reveal exact sales figures, has raised speculation that the gadget — a combined iPod music player, mobile phone and internet browser — is not living up to expectations.

Over the weekend sales of the phones were reported to be slower than expected at stores in Newcastle, Manchester and Birmingham.

Carphone had said that it hoped to sell 10,000 of the devices when the iPhone launched on Friday, while O2 said last week that it had ordered “several hundred thousand” units to sell over the next couple of months.

Mr Erskine’s comments came as he unveiled results for the third quarter to the end of September, which showed the group added 115,000 net new UK customers, taking its total customer base to 17.9 million. This time last year the group added 524,000 new customers.

Guidance for 2007 revenue growth in the unit was unchanged at 15-18 per cent.

The group continued to struggle, though in Germany, in its other key market, with mobile service revenue in the three months to the end of September, down 5.4 per cent year on year to €753 million.

The average revenue per user in the market was €34.9, down 16.2 per cent on a year ago.

However customer additions in the market were 610 000 — the biggest ever amount added in Germany in the third quarter and Mr Erskine said that the “green shoots of recovery” were starting to show in the market where it has struggled with fierce competition.