It’s here. The first U.S. government shutdown in 17 years takes effect on Tuesday morning after Congress and the White House failed to enact spending legislation for the new fiscal year.

The White House Office of Management and Budget issued a memo shortly before midnight ordering executive-branch agencies to implement their plans for closing down in the absence of appropriations.

An estimated 800,000 federal workers now face furloughs indefinitely, while roughly 1.3 million will remain on the job as “excepted” employees because they protect human life and property, or because their positions are paid for by funds outside of congressional appropriations.

A view of the National Mall from the Lincoln Memorial. (Alex Brandon/AP). A view of the National Mall from the Lincoln Memorial. (Alex Brandon/AP).

Federal agencies informed employees last week and on Monday whether they would be furloughed, but official notices are due to arrive on Tuesday now that the shutdown is official.

Federal workers will show up for work on Tuesday regardless of whether they face furloughs, and those who are placed on leave must spend a few hours making preparations for their absences. That means closing down work stations, setting up their out-of-office e-mail alerts, and putting things in order.

More on the Post’s Shutdown FAQ page.

Agencies will eventually pay excepted employees for work they do during the shutdown, but not until Congress approves a new round of appropriations.

The situation for furloughed feds is not so clear, as lawmakers would have to pass legislation that pays them for time they didn’t actually work — through no fault of their own. That’s what legislators did in 1996, but it’s far from certain that today’s Congress would agree to such a deal.

Congress on Monday approved legislation to continue paying members of the military during the shutdown. The White House said President Obama signed the “Pay Our Military Act” late Monday.

The shutdowns during the Clinton administration lasted a combined 26 days and cost the federal government an estimated $1.4 billion, or roughly $2.1 billion in today’s money, according to calculations around that time from the White House budget office. Some of that loss was due to the toll of stopping and re-starting operations, but most of it resulted from the retroactive payments for agency employees, according to a 1997 report by University of Maryland Baltimore County political scientist Roy T. Meyers.

The number of furloughed workers varies widely between agencies. The FBI, for instance, will not place any of its agents on leave during the shutdown, but about 90 percent of Internal Revenue Service employees will be sent home.

The Patent and Trademark Office has said it will remain open for at least a few weeks during the lapse in federal appropriations by relying on reserve fee collections to operate during that time. Fee-generating agencies such as Citizenship and Immigration Services are among those that will fare best under the shutdown, as they derive the least amount of funding from congressional appropriations.

By the numbers: Who will be working during a shutdown

Agency-by-agency impacts of a government shutdown

The federal workforce will not return to full strength until Congress agrees to a new spending plan. No one knows for sure when that will happen, and members of both parties were far from a deal on Monday night.

The last proposal House Republicans approved would have funded the government for a few more weeks while delaying enforcement of the Affordable Care Act’s “individual mandate,” which requires all Americans to obtain health insurance. GOP lawmakers in that chamber proposed additional spending measures in recent weeks that would have undermined the healthcare law, which stands as the Obama administration’s signature achievement thus far.

The Democrat-led Senate roundly rejected each of those plans.

House Speaker John Boehner (R-Ohio) on Monday night called for a special House-Senate committee to meet in the coming days to resolve differences between Democrats and Republicans. Moderate members of both parties have suggested they would be open to a short-term spending bill that continues the last stopgap budget into next year.

Democrats have raised concerns that the shutdown battle will stretch into a more serious battle over the $16.7 trillion federal debt limit. Treasury Secretary Jack Lew has said the government could run out of money to pay its loans in mid-October if Congress does not grant additional borrowing authority.

In a brief phone conversation with Boehner on Monday, Obama reiterated his position that he will not negotiate over the debt limit and that Congress has a responsibility to pay the government’s bills.

To connect with Josh Hicks, follow his Twitter feed or e-mail  josh.hicks@washpost.comFor more federal news, visit The Federal Eye, The Fed Page and Post Politics. E-mail federalworker@washpost.com with news tips and other suggestions.