WASHINGTON — Mailing a letter will soon cost a penny more. The cost of a first-class stamp will rise to 42 cents starting May 12, the U.S. Postal Service said Monday.

The price of the Forever stamp will go up at the same time, meaning those stamps can still be purchased for 41 cents but will remain good for first-class postage after the rate increase takes effect.

The post office has sold 5 billion Forever stamps since they were introduced last April and plans to have an additional 5 billion in stock to meet the expected demand before the May price change, the agency said.

The charge for other services, such as advertising mail, periodicals, packages special services will also change. Changes in the price for Priority Mail and Express Mail will be announced later, the agency said.

Postage rates last went up in May, 2007, with a first-class stamping jumping 2 cents to the current 41-cent rate.

In the past raising postage rates was a long, complex process involving hearings before the independent Postal Regulatory Commission, a process that could take nearly a year.

However, under the new law regulating the post office that took effect in late 2006, the agency is allowed to increase rates with 45-days notice as long as changes are within the rate of inflation for the previous 12 months. The Postal Regulatory Commission calculated that rate at 2.9 percent through January, limiting the first-class rate to an increase of just over a penny.

Under the new law, postal prices will be adjusted annually each May, the Postal Service said. Officials said they plan to five 90 days notice of future changes, twice what is required by law.