The cost of mailing a first-class letter could be increased from 18 to 20 cents today by the U.S. Postal Service Board of Governors, an unprecedented move that would bypass the independent Postal Rate Commission.

It would take at least 10 days for new postal rates to be put into effect if the board adopts them.

The board of governors is scheduled to convene a special meeting at 8 a.m. to discuss postal rates. The Postal Service wants a first-class stamp of 20 cents and other increases for a total additional cost to mailers of more than $3.7 billion.

But the 11-year-old Postal Rate Commission, charged by law with reviewing Postal Service proposals and recommending its own to the board of governors, wants an 18-cent stamp. The rate commission has twice blocked Postal Service attempts to get a 20-cent stamp.

The rate increase is the fifth request since the Postal Reorganization Act of 1970, which created the rate commission. If the board of governors adopts the Postal Service rate increase package today, it would be the first time the board has not approved a rate commission recommendation.

The board favors the 20-cent stamp. But postal observers said yesterday they aren't certain what the governors will do in their meeting today. In order to impose a new rate structure unilaterally, the nine-member board would have to vote unanimously and show that the rate commission's proposal to keep the 18-cent first-class stamp in inadequate.

Among the governor's other options today are: accept the rate commission's earlier recommendation, return it to commission for reconsideration or seek judicial review.

If the board of governors unilaterally adopts the new rates, observers said, it would be open to lawsuits because proponents of lower rates claim the law is unclear on the board's authority to bypass the rate commission.

Last February the commission first rejected the Postal Service's request, saying an 18-cent stamp is adequate to allow the service to break even, and sent its recommendation to the board of governors.

But the board rejected it, saying it would allow the price of a first-class stamp to rise from 15 cents to 18 cents while the rate commission again reviewed the request. At that time, the board of governors said it would immediately seek a permanent increase to 20 cents because of Postal Service needed to break even.

But on June 4, the rate commission rejected for a second time the 20-cent stamp and other increases the Postal Service wanted, creating an unprecedented stalemate.

Postmaster General William F. Bolger has maintained that he needs at least a 20-cent first-class stamp for the Postal Service to break even.

In addition to higher first-class rates, Bolger has recommended raising the price of a postcard from 10 cents to 13 cents and higher parcel post and other rates, resulting in a $3.75 billion increase in rates and fees. The commission has suggested different rates, including a 12-cent postcard.