It comes after 02 said yesterday that the new 3G version of the phone, which went on sale last month, had become its fastest-selling handset ever.
But its status as the only network that offers the phone is hanging in the balance next year.
O2's deal with Apple is believed to run until 2012 and rivals are hopeful that Apple will have partnered with more than just one network by Christmas 2009.
Orange, which already has the iPhone exclusively in France under a three-year deal and also supplies it in a number of emerging markets, is reportedly keen to partner Apple in the UK.
The French-owned network came very close to beating O2 during the bidding process in the UK two years ago.
Apple's contract with O2 is also understood to have a get-out clause, which allows the software company to terminate it if it does not think the phone is being marketed successfully.
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