The Defense Department will furlough roughly 400,000 civilian employees on Tuesday — nearly half of the governmentwide furloughs that will take effect in less than 24 hours if the government shuts down.

More than 800,000 federal civilian employees and as many as 1 million workers will go on temporary unpaid leave beginning Oct. 1 if Congress fails to reach an agreement on funding the government by midnight. Agencies posted their contingency plans online Friday and Monday, as the threat of a shutdown became more likely. Employees who are furloughed, or “non-excepted,” will receive official furlough notices on Tuesday if the government closes.

Federal agencies decide which employees to furlough and which to keep on the job during a government shutdown, though they are required to follow the law’s guidance on definitions. Excepted employees include workers “who are performing emergency work involving the safety of human life or the protection of property or performing certain other types of excepted work,” according to furlough guidance from the Office of Personnel Management. In other words, they aren’t furloughed. Employees who are not funded through annual appropriations are “exempt” from unpaid leave if the government shuts down.

How many employees an agency furloughs during a government shutdown varies, and tends to depend on mission. In some departments, like Veterans Affairs, 95 percent of the workforce stays on the job. At the Housing and Urban Development Department, however, 96 percent of the workforce will go on furlough.

Defense Secretary Chuck Hagel on Monday sent Defense personnel a message on the potential shutdown and the department’s preparations. “Your supervisor will provide more information, but I want you to know that furlough decisions are dictated solely by the law, which only permits us to direct civilians to work if they are required to continue supporting military operations or if they are required to protect DoD personnel and property,” Hagel wrote. “The furloughs are in no way a reflection of the importance of your work, the hard effort you put forth every day, or your dedicated service to our department and our nation.”

Still, the terms “essential” and “nonessential” employees, which are part of the vernacular and not the official government language related to shutdowns, have damaged the already suffering morale of the federal workforce. And for thousands of federal employees, this could be the second round of furloughs in less than a year. About 650,000 Defense civilians were forced to take six days of unpaid leave this summer because of sequestration.

The bulk of the workforce at other agencies, including HUD, the Internal Revenue Service and the Environmental Protection Agency, also will be hit with additional furloughs if the government shuts down. Those agencies shut down for a few days over the summer because of sequestration and most of their employees will not work in a government shutdown.

“This is a workforce which has endured three years of a pay freeze; there has been virtually no hiring, so workloads are increasing dramatically; many already have faced unpaid days because of sequestration; and now they face more unpaid furloughs because of a shutdown that does not need to happen,” National Treasury Employees Union President Colleen Kelley said. “This brinksmanship has got to stop, both for our country and for the dedicated workers who serve the public as federal employees.”

At least one agency will close completely if the government shuts down on Tuesday: the Merit Systems Protection Board. The processing of appeals and other pleadings will be suspended and hearings postponed. “No staff will be available in any MSPB office to answer inquiries during the entirety of a shutdown,” the agency said in a statement. “MSPB e-Appeal Online system also will not be available.” The board had been working through a flood of appeals from furloughs related to sequestration.