The Federal Communications Commission has warned that the two million wireless signal boosters in US homes might contravene new rules designed to prevent interference. But it has hastily rewritten an order that appeared to order existing users to turn the devices off immediately.

The boosters are designed to improve cellphone reception by boosting the local signal to improve the chances of it reaching the nearest cell tower. They are separate to femtocells which relay the cellular signal over your home broadband network.

The FCC says that in principle it’s all for more and more people using boosters, particularly where it can make mobile broadband a viable option for people with limited or no landline options. However, it’s also been investigating the potential for boosters to interfere with wireless networks, especially if the number of users were to drastically increase.

The commission has now decided to lay down technical standards for how boosters work. As part of an agreement with the cellular industry, it’s going to let carriers have some degree of control over the implementation of those standards.

The big change is that once carriers start selling new devices that meet the standards, buyers will not only need to register the booster with their cellular provider, but must get the provider’s permission before using the device.

Originally the FCC noted that “Absent your provider’s permission, you may not continue using your booster.” It’s now clarified that this only applies to new purchases and doesn’t mean existing users must pull the plug unless and until they register and get permission.

However existing users will be forced to switch off their booster if either the wireless provider or the FCC tells them to.

The new system is already looking likely to raise a few problems, with the major networks not yet being ready to explain how the registration and approval will work in practice. It raises some serious concerns about carriers abusing the power and, for example, adding an extra fee for customers who use a booster, or forcing them to get boosters from a particular supplier.

The FCC says letting the carriers control the approval process is the easiest way to make the system work right away without having to go through the lengthy legal process of setting up government regulations. However, it says it retains the option of rethinking this approach if carriers don’t play fair.

(Image for illustrative purposes only and does not represent speculation about any specific booster model.)