The move is set for as early as September and will end Apple's two year exclusive deal with O2.
Crucially, the manufacturer will continue to give O2 exclusivity for the new, faster 3GS version, but will grant distribution of the older 3G handset to another network.
Orange and T-Mobile are known to have already begun pitching for the handset. T-Mobile's call centres have also started telling customers it may stock the iPhone.
T-Mobile and Orange both have existing relationships with Apple, as they offer the iPhone in a number of other countries.