A new report alleges that T-Mobile U.K. is secretly supplying the iPhone 3G unlocked, but only to its highest spending customers who threaten to transfer to another wireless carrier.

According to The Register, T-Mobile U.K. has authorized only 150 phones per week to be issued to certain customers. The phones, which are last year’s model, are officially supported by T-Mobile, although the move is not sanctioned by Apple, which has an exclusive agreement with competing carrier O2.

“T-Mobile has imported an unknown number of iPhone 3G handsets from a European distributor,” the report states, “which it will be supplying to customers paying more than £75 a month if they threaten to leave — the latter clause enforced by allocating the handsets through 50 agents in the retentions department, and limiting those agents to three a week each.”

T-Mobile is not advertising the offering, though, because they do not want to risk upsetting Apple. The carrier has an exclusive contract for the iPhone in the company’s home country of Germany.

Last month, rumors surfaced that the iPhone 3G could be coming to the British T-Mobile after that model’s exclusivity expires with carrier O2. The iPhone 3GS, however, would remain solely on the O2 network.