New York City residents say their so-called “Champagne of drinking water” is more bottom-of-the-barrel prosecco these days.

The Big Apple’s famously delicious tap H2O has been tasting and smelling like mildew and dirt for more than a week, some residents say.

“To me very moldy, but others have described a dirt taste,” said Courteney Corvalan, a Harlem resident who noticed the off-taste last week.

Amy Wilson, in the Upper West Side, tweeted Tuesday, about a “mildew taste” that’s been there for nearly a week.

Another New Yorker tweeted on Monday that her tap water “tastes funny.”

Complaints to 311 have spiked. The standard number of gripes about tap water per day is five, according to the city Department of Environmental Protection.

But in the week from Nov. 18 to Nov. 25, there were more than 150 grumbles to 311 about water quality, with folks describing the flavor or scent as musty, stale, chemical, bitter or metallic — and even “chlorine” or just “sewer,” public records show.

The number of complaints was 10 on Wednesday, according to the DEP.

The majority of gripes came from Manhattan or the Bronx, which, according to the DEP’s water distribution map, receive a mix of tap water from the Croton and Catskill-Delaware reservoir systems.

The Croton Reservoir system, which is more mineral-rich, may be to blame, the DEP said.

“Our water quality scientists believe the more recent taste concerns are related to seasonal changes in the watershed,” said DEP spokesman Edward Timbers.

“As the temperature drops in the Hudson Valley this can affect the natural mixing of water within the city’s Croton Watershed reservoirs,” Timber said. “Operational adjustments have been made to ensure that the best possible water is being delivered to the 8.6 million residents of the city.”

The DEP said the percentage of water coming from each supply system changes on a day-to-day and hour-to-hour basis, and it is tested more than 650,000 times a year to ensure it is safe. Typically, the Croton system reservoirs make up 10 percent of the city’s water supply.

The Catskill aqueduct was shut down six weeks ago for regular maintenance, the DEP said.

Complaints poured in last fall after the city started taking more water from the Croton reservoir system, as it prepared to close one of the two aqueducts in the Catskill-Delaware system for a repair project.