Defense Secretary Leon Panetta formally notified Congress on Wednesday that the Pentagon plans to put civilian defense employees on unpaid leave this year if $46 billion in across-the-board budget cuts take effect on March 1.

The announcement of congressional notification begins a 45-day process that could ultimately lead to unpaid leave for most of the department's 800,000 civilian employees across the United States.

"We expect more than 80 percent to be furloughed," a defense official said.

Panetta, who announced the notification in a message to Defense Department employees as he traveled to Brussels for a NATO meeting, is required by law to advise Congress 45 days before the Pentagon furloughs any workers.

The Pentagon is planning to put most civilian workers on unpaid leave for one day a week for 22 weeks between April and the end of the fiscal year September 30, officials have said.

The military services also were formally notified of the decision on Wednesday and have until early March to ask the Pentagon for exceptions, a defense official said.

The Defense Department will review the requests and approve exceptions by the middle of March, after which workers will receive 30 days notice of the impending furloughs.

The unpaid leave, which will essentially cut the pay of civilian employees by 20 percent, is expected to save up to $5 billion, one of many cuts required as the Pentagon tries to slash $46 billion in spending by the end of the year.

The across-the-board cuts, known as sequestration, are due to go into effect on March 1 unless Congress decides to delay them. They were mandated by the Budget Control Act of 2011 because lawmakers and the White House failed to reach a compromise on alternative spending reductions.

"We are doing everything possible to limit the worst effects on DoD (Defense Department) personnel, but I regret that our flexibility within the law is extremely limited," Panetta said in his message to employees.

"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," he added.

"As a result, should sequestration occur and continue for a substantial period, DoD will be forced to place the vast majority of its civilian workforce on administrative furlough," Panetta said.