A breakdown in the city’s 911 system blocked nearly 1,700 emergency calls for service for seven hours on Tuesday, affecting as many as 400 New Yorkers, according to the NYPD.

A glitch in the system’s Bronx call center, one of two in the five boroughs, disabled calls to 911 operators in parts of the Big Apple between 3:13 a.m. and 10 a.m. — with sources attributing the issue to a possible software failure and not a malicious cyber attack.

The city maintains another Public Safety Answering Center in Brooklyn that was not affected.

About 1,700 callers “were impacted,” City Hall spokesman Jeremy Edwards told The Post on Wednesday, adding that “we believe up to 400 New Yorkers may have been unable to connect with a 911 call taker” during the breakdown.

He said the problem also resulted in operators not being able to hear callers.

A total of 5,659 calls for EMS, FDNY and NYPD response came into the 911 system during the time it was down.

“The issue has since been resolved, and PSAC II is running normally,” Edwards said in a statement. “All 911 calls have been connected and responded to by a call taker since 10:19 a.m. yesterday.

“We performed intensive overnight testing on the system, and are monitoring the call delivery, traffic paths, and volume 24/7 in close coordination with our technical providers to ensure there are no additional service impacts,” Edwards said.

The Big Apple overhauled its antiquated 911 system beginning in 2004, under then-Mayor Michael Bloomberg.

But the project was fraught with delays, cost overruns and persistent mismanagement that boosted the cost over $2 billion, according to a 2015 report by the city Department of Investigation.

On Wednesday, City Hall officials said they were still trying to determine the cause of the concerning, hours-long glitch.

“Our investigation into the circumstances of this incident is ongoing,” Edwards said. “We are focused on understanding exactly what happened, ensuring that affected callers receive the appropriate follow-up and taking all necessary steps to prevent this from happening again.”