New York City has stalled work on its so-called third water tunnel, which would provide a crucial back-up H2O source for millions of outer-borough residents.

Most of the massive engineering project has already been completed — but construction funding for branches serving Brooklyn and Queens has been reallocated, according to The New York Times.

Mayor de Blasio’s spokeswoman Amy Spitalnick said work on the tunnel can still move forward.

“The city remains committed to getting this project completed as quickly as possible,” she said.