The top U.S. telecommunications regulator said on Wednesday he is "deeply troubled" by a Verizon Communications Inc (VZ) announcement that high-speed wireless customers who subscribe to older unlimited data plans might experience slower speeds starting Oct. 1.

Verizon stopped offering unlimited data plans in 2012 and last week said it will soon begin slowing services for the top 5 percent of data users who are on unlimited plans in places where the network is experiencing high demand.

Federal Communications Commission Chairman Tom Wheeler, in a letter on Wednesday to Verizon Wireless President and CEO Daniel Mead, said he was "deeply troubled" about that plan.

"'Reasonable network management' concerns the technical management of your network; it is not a loophole designed to enhance your revenue streams. It is disturbing to me that Verizon Wireless would base its 'network management' on distinctions among its customers' data plans, rather than on network architecture or technology," Wheeler wrote.

"I know of no past Commission statement that would treat as 'reasonable network management' a decision to slow traffic to a user who has paid, after all, for 'unlimited' service."

In the letter, Wheeler also posed to Mead several questions about the plan, including the rationale to treat consumers differently depending on their data plans, the reasons behind extending speed reduction from the slower-speed 3G network to the more efficient high-speed 4G LTE network, and how Verizon would justify the plan under its current regulatory obligations.

Verizon representatives did not immediately have comment.

Verizon's announcement came as wireless carriers attempt to shift data-hungry subscribers onto tiered plans, which charge customers for individual data packages.

Verizon's policy is already in effect for unlimited subscribers on the 3G network but will be expanded to its higher-speed 4G network in October.

(Reporting by Alina Selyukh; Editing by Jonathan Oatis and Grant McCool)