WASHINGTON - The cost of sending a first-class letter will rise today by 3 cents, to 37 cents, as part of a broad postage increase. A postcard will cost 23 cents to mail, or 2 cents more.

The independent Postal Rate Commission approved the changes in March.

Postal officials say the increases are intended to help the financially struggling service recover from the economic downturn, declining business and hundreds of millions of dollars in expenses incurred after the September terrorist attacks and the discovery of anthrax in the mail.

Postmaster General John E. Potter has said the service will not increase rates again before 2004. The previous increase in first-class stamps was 1 cent in January 2001, to 34 cents.

The service lost about $1.7 billion last year, officials say, and it could lose as much this year. It has cut about 30,000 jobs and halted new construction.

In April, Potter sent Congress a plan for overhauling the service, including more autonomy to consolidate facilities, phase in rate increases and reduce six-day delivery. The service also proposed that it become a "commercial government enterprise," with the flexibility and structure of a private business.

The service receives no tax money, but it works under congressional guidelines, and changes require congressional approval.