Kenyan mobile phone firms have been ordered to install equipment to allow the telecommunications regulator to monitor activity on their networks.
It means the Communications Authority of Kenya (CA) would have the ability to listen to calls, read messages and view financial transactions.
But the body says the move is not about mass surveillance and is necessary in its fight against counterfeit phones.
Such handsets cannot be tracked and are often used by criminals.
The country’s biggest telecoms operator, Safaricom, says it has “strong reservations” about the move.
Stephen Chege, Safaricom’s corporate affairs director, told Reuters that the system needed to be “subjected to the relevant public debate as it touches on confidential communications belonging to our customers”.
The BBC’s Wanyama wa Chebusiri in the capital, Nairobi, says the regulator’s proposal would see more than 30 million mobile phone owners in Kenya lose their privacy and confidentiality.
The issue of counterfeit phones has been a main concern for authorities.
Many are imported from Asia, and regulators say are difficult to track as they lack authentic International Mobile Equipment Identity (IMEI) numbers.
In its letter to the three biggest mobile phone providers, Safaricom, Airtel and Orange, the regulator asks for their contractor to be allowed on to the companies’ sites to install a snooping device, called a Device Management System (DMS), from next week.
The regulator cracked down on counterfeit mobile phones in the past – switching off 1.5 million fake phones in 2012.
But it says consumers are still buying them and so this is part of its efforts to tackle the problem.
Kenya denies mass mobile phone surveillance plan