Mayor de Blasio pledged Tuesday to hire private security guards to pick up the slack on Rikers Island amid continued staff shortages in the Correction Department ranks — even though a decades-old law appears to bar the city from outsourcing jail jobs.

As conditions in the overcrowded jail remain dire, de Blasio said his administration is working on contracting a private firm to take over certain duties for correction officers, some of whom say they are regularly told to work double or triple shifts.

“We’re going to bring in private security,” de Blasio said. “It’s going to provide a lot of relief and a lot of ability for Commissioner Schiraldi to get officers where he needs them most to speed things up in Rikers and improve conditions.”

The mayor did not elaborate on how many personnel he hopes to hire or what their duties would be.

But the privatization push could run into trouble because of a bill signed into law by Gov. George Pataki in 2002.

The bill, a product of lobbying by ex-correction officers union boss Norman Seabrook, codified that guarding inmates in city jails can’t be “delegated, transferred or assigned in whole or in part to private persons or entities.”

Benny Boscio, the current president of the Correction Officers’ Benevolent Association, said the 2002 measure makes clear the mayor’s proposal is “illegal.”

“The law is written to protect inmates from the barbaric hands of privately run prisons. De Blasio is putting his own interests above the safety of all those on Rikers Island once again,” said Boscio, whose union has called on the mayor to instead hire more correction officers to address the volatile situation on the island. “This is de Blasio attempting to cover up years of intentional neglect, failing to hire any [jailers] and leaving Rikers to rot until it closes.”

A City Hall spokesman did not return a request for comment on how the city would be able to get around the 2002 law.

According to sources in the correction officers’ union, de Blasio’s administration has already tried to hire a private security firm for Rikers’ duties.

The contract would’ve tapped Rockland County-based Brosnan Security to handle “exterior and perimeter security” on the island, allowing about 150 correction officers to be reassigned to guard inmates inside the jail, the sources said.

But the sources said the security firm pulled out after it was approached by union reps who told the company the contract would not survive a legal challenge.

A Correction Department official disputed that claim, saying that Brosnan walked away because the contract floated by the administration was “too short-term.”

“They came in for a visit, crunched the numbers, decided it’s not going to work for them. But this is still something the department is still looking to do,” the official said. “They just weren’t the right fit.”

While plans for private security are up in the air, Rikers remains in crisis.

Another Rikers inmate died in custody Sunday — at least the 11th causality on the island this year.

A perilous mixture of Correction Department staffing shortages and inmate overcrowding has resulted in jail conditions deteriorating catastrophically. State legislators who toured Rikers earlier this month said some inmates were being held without access to food, beds or medication for days on end, while the jail’s hallways were littered with human feces, urine and dead cockroaches.

De Blasio has blamed the chaos on hundreds of correction officers calling out sick without proper documentation, and his administration filed a lawsuit against the officers’ union this week, charging that the union is encouraging mass absenteeism.

Inmate overcrowding is an issue de Blasio claims will be addressed by the “Less Is More Act,” a bill signed into law by Gov. Hochul last week that could release hundreds of inmates locked up on minor parole violations.

However, most components of that bill won’t take effect until next year.

Prison reform advocates have called on de Blasio to, in the meantime, free parole violators via a supervised release program known as 6A.

But de Blasio has bucked those requests.

“We’re not looking at 6A right now,” he said on WNYC last week.