WASHINGTON —  Preparing for possible war against Iraq, the Marine Corps has taken the unusual step of stopping all Marines from leaving the service for the coming 12 months, officials said Thursday.

The decision was announced to all Marines in an internal message Tuesday from Gen. James Jones, the Marine Corps commandant. He said it applies to active-duty as well as reserve Marines and is effective Jan. 15 through Jan. 31, 2004.

The last time the Marines took this action, known as "stop-loss," was during the buildup to the 1991 Gulf War, said Capt. Gabrielle Chapin, a spokeswoman at the Combat Development Command at Quantico, Va.

Although the order applies to the entire Marine Corps, which has 173,000 men and women on active duty and 100,000 in the reserves, some exceptions can be made, including for those who already have a retirement date of April 1 or earlier. Commanders also can consider exceptions for personal hardship and other circumstances.

Last fall the Army issued "stop loss" orders for nearly all its Special Forces soldiers, who are heavily engaged in the war in Afghanistan, although the action did not apply to the rest of the force. The Marines are the only ones to apply a "stop loss" action to all service members.

In addition to the approximately 1,000 Marines from the 1st Marine Expeditionary Force who recently arrived in Kuwait in anticipation of a possible war against Iraq, thousands more are either heading to the Gulf region from U.S. bases or are preparing to do so in coming weeks.

Specific figures are classified, but Jones said Wednesday in remarks at the National Press Club that if there is a war in Iraq, the Marines likely would contribute between 65,000 and 75,000 troops — mostly from the 1st Marine Expeditionary Force at Camp Pendleton, Calif.

The Pentagon's Iraq war plan calls for the use of up to 250,000 troops from all services.