Residents of a large section of Prince George’s County were told Monday night to stockpile water immediately in anticipation of an emergency shut-off of their supply for days during the hottest period so far this summer.

The announcement came after authorities detected signs that part of a major water main, which supplies at least 100,000 people, was about to fail and required immediate replacement.

The Washington Suburban Sanitary Commission, which provides water to the affected area, said the main could be closed down as early as Tuesday afternoon and told people to stock up before then. One spokesman said that once cut, service through the 54-inch main could be out for as long as four days.

The problem could not have occurred at a worse time, with Washington’s high temperatures soaring well into the 90s and expected to remain there through the workweek.

The area supplied by the pipe, according to a map provided by WSSC, includes much of the southern and western parts of Prince George’s between Joint Base Andrews and the Potomac River and from the boundary with the District to several miles south of the Capital Beltway.

Andrews was one area in the zone served by the pipe, along with the communities of Morningside, Hillcrest Heights, Camp Springs, Forest Heights, Temple Hills and Oxon Hill, in addition to National Harbor, the WSSC said in a statement.

It was not immediately clear whether the affected area included office buildings and, if so, what provisions would be made for those working there. WSSC said the county would work with hospitals, nursing homes and others to ensure access to water.

Also unclear was the extent to which water was used for cooling equipment in office and commercial facilities in the affected area.

Ample water consumption — remaining hydrated — is one of the major recommendations by health officials for coping with the heat.

WSSC spokesman Jim Neustadt said the prestressed concrete cylinder pipe involved was essentially the only water source for the area served, making it necessary for people “to fill up on water” ahead of the shutdown. Authorities urged that residents take such steps as filling bathtubs to keep water on hand.

Before the flow of water through the pipe is cut, WSSC would try to fill up its storage facilities downstream so that some water could be delivered through smaller pipes, said Jerry Irvine, another utility spokesman.

But that stored water would be enough for only about 12 to 15 hours of normal use, Neustadt said. Another major concern would be fire protection, he said.

Officials said that once replacement of the pipe began, it would be too late to stock up on water and that residents would then need to curb usage.

“We are asking people to fill up, go to stores, try to get water any way they can,” Neustadt said, and then “to use it judiciously.” One WSSC official said at a news conference, “We fully expect there will be those who will be fully out of water.’’

The emergency announcement came after a sudden spike in alarms from a system that is designed to monitor the mains for signs of failure. Neustadt said he understood that the spike had occurred within a period of about 24 hours and included more signals than would occur at random.

The utility has been installing acoustic equipment inside its largest water mains to detect the onset of any snapping of the steel wires that reinforce the concrete. The snapping is a sign that the pipe is failing; the monitoring is designed to make it possible to replace affected portions of the pressurized pipe before a potentially catastrophic blowout.

This is at least the third time in the past several years that acoustic equipment installed in a pipe has alerted WSSC to a potential break, allowing for a preemptive shutdown.

One such shutdown occurred in the Rockville area this spring, utility officials have said. In 2010, water restrictions were imposed over the Fourth of July weekend after a 96-inch pipe was shut down in Potomac.

At least two forms of prestressed pipe have been manufactured. In both, the wire is wrapped around the pipe. In the older form, the wire is wrapped around a steel cylinder, which in turn covers a concrete core. In the more recent form, the wire is wrapped directly around the concrete core. The wire is used to produce compression to offset stresses in the pipe.

The utility is still reviewing the March break of a 60-inch pipe in Chevy Chase, where the force of the blast blew a crater 20 feet deep in a side street off Connecticut Avenue, creating a 40-foot-high geyser. In that case, utility officials said, no warning was received because the break occurred near a joint where there was no wire or monitoring.

The main that triggered the latest alarms is inside the Beltway between Suitland Parkway and Forestville Road. The WSSC said it is in a relatively inaccessible spot, and officials said a road was being constructed Monday night to bring in repair equipment.

Irvine said that repair equipment should be on site sometime Tuesday morning and that the main will be taken out of service. Another WSSC official said the replacement could begin as soon as Tuesday afternoon.

Once the main is out of service, authorities said, the only water available in the system would be in the storage facilities and whatever remained in the portion of the pipeline downstream of the replacement area.

The WSSC’s 350 miles of prestressed concrete cylinder pipe form the backbone of its distribution system for 1.8 million people in Montgomery’s and Prince George’s counties.

Katie Shaver contributed to this report