Embattled power company Con Edison completely backtracked Monday — admitting that a power cable caught fire and failed at its West 65th Street station triggering the massive blackout that left a large swath of Manhattan’s West Side in the dark for five hours.

“In this case, primary and backup relay systems did not isolate a faulted 13,000-volt distribution cable at West 64th Street and West End Avenue,” the utility said in a statement.

A Con Ed spokesman later confirmed to The Post that the cable caught fire, setting off a series of systemwide failures — with a key safety system at the substation failing to shut off power to the cable, which then allowed the problem to quickly spread from Chelsea to the Upper West Side, including Rockefeller Center, Times Square and the Broadway theater district.

“While the cable fault was an initiating event,” Con Ed added in its statement, “the customer outages were the result of the failure of the protective relay systems.”

The problem then spread to the utility’s major substation on West 49th Street, where its protective systems kicked in, shutting down power to more than 72,000 customers. The number of New Yorkers impacted was likely far higher as many buildings operate off of a single meter.

But that was a very different story from the one utility President Timothy Cawley told the public a day earlier at a Sunday afternoon press conference with Mayor Bill de Blasio.

There, he discounted the possibility that the single, burned-out underground cable triggered the blackout, calling the theory a “nonstarter,” and described problems with underground cabling as routine.

“They fail – we have a lot of them and they, they fail on occasion, and we have a lot of maintenance and replacement programs in place to mitigate that, to sort of identify where the most risky areas – the riskiest areas are,” he said.

However, Cawley also added it would likely take “weeks” to probe the root cause the outage.

The utility said that its investigation into the Saturday evening blackout continues.

Con Ed’s about-face left de Blasio — freshly back from Iowa — demanding an explanation.

“The city that never sleeps cannot be left in the dark. While I appreciate that Con Edison released their preliminary findings so quickly, I am troubled that one of the few factors Con Edison initially ruled out, the 13,000 volt cable, has been determined to be the catalyst of the outage,” Hizzoner said in a statement. “Our city cannot be left in the dark like this ever again, and we will continue to push Con Edison for a full accounting of this incident to ensure they are taking necessary steps to protect all New Yorkers.”

Meanwhile, an expert told The Post that the failure of protective systems at the 65th Street station could allow problems to quickly ripple across the power grid.

“If the relay doesn’t trip fast enough, the electrical problem can spread like a virus,” explained Karl Rábago, the executive director of the Pace University law school’s energy and climate center. “It’ll build and spread until the system trips.”

He said that Con Ed’s new spin on what caused Saturday night’s blackout only raises new questions for him about the utility and the fragility of its system.

“It strikes me as a little obtuse that the way they described Saturday’s blackout leads you to focus on the failure of the protective relay and not on the root cause of the cable failure,” he added.