Advertisements that make airfares seem enticingly low will soon lose that asterisk pointing to a dense paragraph of additional taxes and fees that make a cheap ticket much more costly.

Beginning Jan. 24, the Transportation Department will enforce a rule requiring that any advertised price for air travel include all government taxes and fees. For the last 25 years, the department has allowed airlines and travel agencies to list government-imposed fees separately, resulting in a paragraph of fine print disclaimers about charges that can add 20 percent or more to a ticket’s price.

But with airlines now promoting fares on Web ads, Facebook and Twitter, and adopting a menu of fees for services that used to be part of the ticket price, the government decided it was time for a change so travelers have a clearer sense of the total price they must pay. (The price will not include baggage fees, though, because they are optional.)

“Requiring all mandatory charges to be included in a single advertised price will help consumers compare airfares and make it easier for them to determine the full cost of their trip,” Bill Mosley, a department spokesman, said by e-mail in response to questions about the rule.