WASHINGTON —  In an action branded a backdoor draft by critics, the military over the past several years has held tens of thousand of soldiers, sailors, airmen and Marines on the job and in war zones beyond their retirement dates or enlistment length.

It is a widely disliked practice that the Pentagon, under new Defense Secretary Robert Gates, is trying to figure out how to cut back on.

Gates has ordered that the practice — known as "stop loss" — must "be minimized." At the same time, he is looking for ways to decrease the hardship for troops and their families, recruit more people for a larger military and reassess how the active duty and reserves are used.

"It's long overdue," said Jules Lobel, vice president of the Center for Constitutional Rights, and lawyer for some in the military who have challenged the policy in court. "It has created terrible problems of morale."

Gates has asked the chief of each service branch for a plan by the end of February on how they would rely less on stop loss.

The authority has been used off and on for years and was revived by all services to some extent after the attacks of Sept. 11, 2001.

As an example, the Army revived it in early 2002 to keep people with some skills or specialties deemed critical to the fight against terrorism and later used it to retain whole units, according to an Army chronology of the policy.

Pentagon officials provided no figures on how many people the policy has affected. Yet just in the Army, it is in the tens of thousands.

The Navy stopped a few hundred sailors from leaving in the year after the terrorist attacks and used the policy again after the U.S.-led invasion of Iraq in 2003.

The Marine Corps used it from January through August of 2003 and at the high point had some 3,400 active duty troops and 440 reservists held in service under the authority, said 1st Lt. Blanca E. Binstock, a spokeswoman.

The Air Force did not have statistics immediately available.

The Defense Department says the main reason for the policy is to keep units whole for deployments.