WASHINGTON — Federal regulators on Thursday proposed a set of privacy rules for Internet service providers that would significantly curb the ability of companies like Comcast and Verizon to share data about their customers’ online activities with advertisers without permission from users.

In the proposal, before the Federal Communications Commission, the agency’s chairman, Tom Wheeler, called for broadband service providers to disclose clearly how data may be collected about users’ online browsing and other activities. The plan also called for the companies to bolster the security of customer data.

The proposal, if approved, would for the first time establish privacy rules for the companies that manage the traffic of the web and would create some of the strongest privacy regulations for any segment of the technology and telecommunications industries. They represent the first major regulatory action involving broadband providers after the F.C.C.’s declaration last year that high-speed Internet carriers should be treated like utilities.

In a statement, Mr. Wheeler said that since Internet service providers handle all network traffic, the companies have a “broad view of all of your unencrypted online activity.” He added that “even when data is encrypted, your broadband provider can piece together significant amounts of information about you — including private information such as a chronic medical condition or financial problems — based on your online activity.”