O2, until this week Apple's sole partner for the iPhone in the UK, is struggling to supply customers with the smartphone due to lack of available supplies.

Newspapers and enthusiast sites have reported a drop in the number of phones available, particularly the 3GS model, in stores and online.

One Carphone Warehouse employee told Mobile magazine: "They are being delivered sporadically - we just haven't been able to get hold of them for four or five weeks. Stock has not returned to normal levels since the launch of the new model." Stocks of the original 3G are still widely available.

The Telegraph meanwhile, reports that the majority of O2 shops across the UK have been without the 3GS models for the last two weeks, while the O2 website has suffered similar stock shortages. "We continue to see extremely high levels of demand for the iPhone which means it comes in and out of stock very quickly and will be why the website hasn't had any since Monday," and O2 spokesman told the newspaper.

The Telegraph estimated that about 30,000 customers have bought the iPhone from O2 within the last two weeks.

Mark Mulligan, vice president and research director of Forrester Research claimed Apple still has growing pains when it comes to supplying the phone. "There is no doubt that Apple is still adjusting to being a player in the mobile phone market. It's a different game than what it's used to... However I expect it will be building up its supply chain in advance of becoming available on multiple networks in the UK."

"I don't think it would expand if it couldn't meet consumer demand, as that would lead to extreme consumer dissatisfaction - which compared to its peers, Apple is usually good at avoiding."

With Orange this year and later in 2010 Vodafone able to officially offer the iPhone, reports suggest the limited stock will be shared amongst mobile phone network providers putting the pressure on O2.

Orange will need sufficient handsets to launch the iPhone successfully sometime before Christmas.

Betting site Paddy Power believes Orange, who are on target to buy T-Mobile to become the UK's largest mobile phone company later this year, are 11-10 favourites to sell most iPhones in 2010.