The Federal Communications Commission is set to propose about $200 million in fines against four major cellphone carriers for selling customers’ real-time location data, according to three people briefed on the discussions.

The penalties would be some of the largest the agency has imposed in decades and represent the first action it has taken on the issue, though privacy advocates and critics in Congress say even that response is inadequate. The amount is not final, and the companies — AT&T, Sprint, T-Mobile and Verizon — will have the opportunity to respond and argue against the fines.

The trade in location data has emerged as a sensitive privacy issue because it affects hundreds of millions of people and can reveal intimate details about their lives, including personal relationships and visits to doctors. “It puts the safety and privacy of every American with a wireless phone at risk,” Jessica Rosenworcel, a Democratic commissioner at the F.C.C., said in a statement last month about the agency’s investigation.

Sale of the data is widespread among app makers and other technology companies, but the telecommunications sector is subject to more stringent laws protecting customers’ confidentiality.