WASHINGTON — Defense Secretary Leon Panetta told Congress on Wednesday that if automatic government spending cuts kick in March 1 he likely will have to shorten the workweek for the “vast majority” of the Defense Department’s 800,000 civilian workers.

They would lose one day of work per week, or 20 percent of their pay, for up to 22 weeks.

Panetta also said the across-the-board spending reductions would “put us on a path toward a hollow force,” meaning a military incapable of fulfilling all of its missions.

In a written message to employees, Panetta said he notified members of Congress on Wednesday that ifthe White House and Congress cannot strike a deficit reduction deal before March 1 to avoid the furloughs, all affected workers will get at least 30 days’ advance notice.

The furloughs would be part of a broader plan the Pentagon is preparing in order to cut $46 billion through the end of this budget year, which ends Sept. 30. More cuts would come in future years as long as the automatic government spending cuts, known as sequestration, remained in effect.

“In the event of sequestration we will do everything we can to be able to continue to perform our core mission of providing for the security of the United States, but there is no mistaking that the rigid nature of the cuts forced upon this department, and their scale, will result in a serious erosion of readiness across the force,” Panetta wrote.

Adding his voice to the budget debate, Secretary of State John Kerry said the fiscal impasse is a serious threat to American credibility around the world.

“Think about it: It is hard to tell the leadership of any number of countries that they must resolve their economic issues if we don’t resolve our own,” Kerry said Wednesday in a speech at the University of Virginia.

Kerry, in his first major address as secretary of state, said that an internationally engaged United States fills a vacuum that would otherwise be occupied by “those whose interests differ dramatically from our own” and gives U.S. businesses a leg up in global competition.

“Every time a tough fiscal choice looms, the easiest place to point fingers is at foreign aid,” Kerry said. He added that all foreign-policy spending amounts to a little more than 1 percent of the federal budget.

The forced budget cuts would reduce State Department operations by $850 million and foreign assistance by $1.7 billion, according to a letter Kerry sent Congress on Feb. 11. Before those cuts, President Barack Obama requested a total of $51.6 billion for fiscal 2013 for the State Department and the U.S. Agency for International Development.

In a hat tip to one of the State Department’s fiercest critics, Kerry quoted a former colleague, Republican Sen. Lindsey Graham of South Carolina, as he argued that “deploying diplomats today is much cheaper than deploying troops tomorrow.”

“It’s national security insurance that we’re buying,” Kerry quoted Graham as saying.

Panetta was flying Wednesday to Brussels to attend a NATO defense ministers meeting. Spokesman George Little said Panetta would tell his counterparts that acrossthe-board budget cuts will hurt not only the U.S. military but also the ability of NATO to respond to crises.

Little said the Pentagon also is discussing the possibility of not being able to send military units on planned rotations to various places around the world. In anticipation of cuts, the Pentagon has already decided not to send one aircraft carrier back to the Persian Gulf, reducing the U.S. presence there to one carrier.

Pentagon officials have said their furloughs would be structured so that nearly all 800,000 civilian workers lose one day of work per week for 22 weeks, probably starting in late April.

“We are doing everything possible to limit the worst effects on [Defense Department] personnel but I regret that our flexibility within the law is extremely limited,” Panetta said. “The president has used his legal authority to exempt military personnel funding from sequestration, but we have no legal authority to exempt civilian personnel funding from reductions.”

The Pentagon has begun discussing details of the furloughs with defense worker union officials.

House Speaker John Boehner put the blame on Obama and said he agrees with Panetta that automatic spending cuts would devastate the military.

Boehner, R-Ohio, released a copy of Panetta’s letter formally notifying Congress that the Pentagon will have to consider furloughing a large portion of its civilian work force if sequestration kicks in.

“The furloughs contemplated by this notice will do real harm to our national security,” Panetta wrote in his congressional notification letter, adding that it would make troops less ready for combat and slow the acquisition of important weapons.

“Overall, sequestration will put us on a path towarda hollow force and inflict serious damage on our national security,” Panetta wrote.

The only civilian Pentagon workers who would be exempt from furloughs would be Senate-confirmed political appointees such as the defense secretary and deputy defense secretary, as well as a relatively small number of workers deemed essential to protect the safety of defense property and personnel.

“We will except citizens deployed,” Pentagon Comptroller Robert Hale said. “We will not furlough civilians deployed in combat zones. We will not furlough civilians who are required to maintain safety of life or property, but only to the extent that they have to do that to maintain life and property.”

Panetta said the administration is still working with Congress to avoid automatic budget cuts by reachingagreement on a deficit reduction plan.

Unless lawmakers and Obama can agree on an alternative, the spending reductions will kick in. About half the cuts will affect defense spending, requiring about $46 billion in reductions in the seven remaining months of this fiscal year and about $500 billion over a decade.

The Pentagon estimates furloughs of civilian employees would cost them as much as $4.86 billion in pay in the fiscal year that ends Sept. 30.

Virginians would see the biggest drop in payrolls, with $660.9 million lost, according to the estimate released by the Defense Department. Thatwould be followed by civilian employees in California, which would see a $419.7 million decline; Maryland, losing $359.3 million; and Texas, losing $290.9 million.

Military leaders said last week that allowing the spending reductions to take effect would mean less training for Army personnel and fewer purchases of Navy vessels and Air Force fighter jets.

Separately, the Army said sequestration, combined with stopgap defense funding already in effect, may trigger furloughs or job cuts for more than 302,000 of its employees and those of contractors and businesses that provide services, according to a planning document obtained by Bloomberg News.

Nationwide, the job effects and reductions in base operations and improvements would cost the economy $15.4 billion, the service said.

The hardest-hit state economically would be Texas, followed in order by Alabama, Pennsylvania and Virginia, according to the Army.

Texas, home to some of the biggest Army posts such as Fort Hood, may lose $2.4 billion from the service’s spending reductions, according to the service. Almost 35,000 jobs would be affected, including about 30,000 civilian workers.

The Army said it has begun cutting its contractor work force, firing temporary employees and canceling depot maintenance, among other actions, to help offset an expected $18 billion budget shortfall in the fiscal year that began Oct. 1.

Information for this article was contributed by Robert Burns, Donna Cassata, Lolita C. Baldor, Bradley Klapper of The Associated Press and by Tony Capaccio, Brendan McGarry, Nick Taborek, Nicole Gaouette of Bloomberg News.

Front Section, Pages 1 on 02/21/2013