WASHINGTON — The Army is reducing the rank of a brigadier general at the center of a sexual misconduct case by two grades for his pending retirement, in a rare move that will slash his benefits and force him to retire as a lieutenant colonel.

Army Secretary John McHugh announced the decision Friday, saying that Brig. Gen. Jeffrey A. Sinclair “displayed a pattern of inappropriate and at times illegal behavior both while serving as a brigadier general and a colonel.”

Sinclair is due to retire in late summer.

McHugh’s move comes three months after Sinclair pleaded guilty at a court martial to adultery and conducting inappropriate relationships with two other women.

Rep. Jackie Speier, D-Calif., a member of the Armed Services Committee and one of the key voices in the House pushing for stronger action on sexual assault in the military, said Friday that McHugh’s action was inadequate.

“This punishment is in no way proportional to the laundry list of serious offenses Sinclair pled guilty to,” Speier said. “The final outcome of this case can only be seen as further evidence that the military justice system is broken and that the chain of command cannot adequately address the gravity of these offenses.”

Sinclair had a three-year affair with a female captain who accused him of twice forcing her to perform oral sex on him. The former deputy commander of the 82nd Airborne Division was originally brought up on sexual assault charges punishable by life in prison. Sinclair was spared prison but fined $20,000 and issued a reprimand.

He was believed to be the highest-ranking U.S. military officer ever court-martialed on such charges. And, the Army said this is the first time in a decade that the service has reduced a retiring general officer by two ranks.

“Gen. Sinclair has consistently taken responsibility for his mistakes and agreed to a reduction in retirement benefits,” his attorney, Richard Scheff, said in an emailed statement. “He is a highly decorated war hero who made great sacrifices for his country, and it’s right that he be permitted to retire honorably.”

While retirement benefits are mandated by federal law, there is a requirement that an individual must have served satisfactorily in rank before receiving those benefits, McHugh said in a statement. McHugh determined that Sinclair’s actions as both a one-star general and colonel provided enough evidence to reduce his rank in retirement by two grades. McHugh added that he was prevented by federal law from taking any further action against Sinclair.