A group of 15 senators is calling on the Army to change its policy regarding prior-enlisted captains and majors who are being forced to retire at their highest enlisted rank.

In a letter to Army Secretary John McHugh, the lawmakers, led by Sen. Patty Murray of Washington and Sen. Johnny Isakson of Georgia, said they have "grave concerns" over the treatment of these officers.

The Army this year selected about 1,100 captains and more than 500 majors for involuntary separation as part of an ongoing draw down to reach an end-strength of 490,000 soldiers by the end of fiscal 2015. Among those officers were soldiers who had previously served as enlisted troops before earning their commission.

Forcing these officers to retire at their highest enlisted rank could cause them to lose as much as $1,000 a month in some cases, the senators wrote. That could mean a "significant decrease" in retirement pay worth just over $1 million over a 40-year retirement in the case of a captain who's forced to retire as a sergeant first class, according to the senators' letter.

"These former non-commissioned officers answered the Army's call for volunteers to attend Officer Candidate School as the Army expanded its officer corps to fight the wars in Iraq and Afghanistan. Now, despite having served for years as commissioned officers and rising through the ranks to become captains and majors, these dedicated soldiers will soon be forced to retire at their highest previous enlisted rank," the letter states. "To demote these soldiers in retirement is an injustice that devalues their service and will materially disadvantage them and their families for the rest of their lives."

In a June 17 directive, McHugh reduced to eight years the minimum number of years of active commissioned service required before an officer is eligible for retirement, the shortest amount of time allowed under federal law.

Before that directive, federal law required these officers to serve at least 10 years of commissioned service before being eligible to retire as officers. Any further changes to the minimum requirement will require changing the law, which will have to come from Congress.

Of the captains and majors selected for separation by this year's boards, 101 captains will not have obtained the minimum amount of commissioned service to retire as an officer, according to data compiled after the boards.

Of those 101 captains, only 22 can ask the Army secretary to defer their retirement date from April 1 to June 30, giving them time to have served eight years of commissioned service, according to the data.

The 150 majors selected have all obtained the minimum eight years of commissioned service, according to the same set of data.

In their letter to McHugh, the senators called on the Army to change its policy to "delay the mandatory retirement date of affected soldiers until the first month after they become eligible to retire as commissioned officers."

"For many of the affected soldiers, this would extend their time in service by only a few months," the senators wrote.