Mayor Bill de Blasio on Tuesday voiced doubts over calls to close down violence-plagued Rikers Island and build borough-based jails, warning that doing so could take too long and cost too much money.

Shuttering the island jail complex, which has housed much of the city’s inmates since the 1930s, is a solution proposed in a speech last week by Council Speaker Melissa Mark-Viverito and endorsed this week by Gov. Andrew M. Cuomo.

“Now people will say, ‘Well that’s too big a challenge for us to tackle.’ I say baloney,” Cuomo told NY1 Tuesday morning.

But while de Blasio said he welcomes the state’s help, he said such a plan to replace the jail complex with an average daily head count of about 10,000 inmates would cost “billions and billions.”

“I want to be real with people,” he told reporters at an unrelated news conference in Red Hook, Brooklyn. “It’s a very big, costly, complicated endeavor.”

The mayor added: “Where would we then put the inmates? How would we pay for it? Would it be logistically viable? We don’t have any of those answers right now.”

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Cuomo shrugged off worries of resistance from neighborhoods to jail facilities in their vicinity.

“You’ll never find a site where everybody is happy, but if you needed a solution that made every New Yorkers happy, trust me, you would have no solution,” Cuomo told WCBS 880 radio.

New technology and design would make jails safer for inmates and guards alike, he said. Most of the inmates are pretrial detainees, innocent under the law, he noted.

De Blasio said that such a plan could take 5 or 10 years, too long to wait for needed reforms. He called the idea “a noble concept but one that will cost many billions of dollars, and we do not have a viable pathway to that at this point.”

The U.S. Justice Department, which sued the city over what it called a “deep-seated culture of violence” at Rikers, last year settled the case after being promised a range of reforms to the jail’s culture, use-of-force policies and surveillance-camera deployment.