The de Blasio administration is illegally shipping homeless families off to unsuspecting, upstate towns with a year of free rent — forcing those municipalities to pick up the rest of their living expenses, Broome County officials said Thursday.

At least five families have been sent to Broome in recent months, and officials there issued an ultimatum — either end the Special One Time Assistance Program, known as SOTA, or face civil and possibly even criminal litigation.

“We’ve put New York City on notice,” seethed Broome County Executive Jason Garnar, one of several officials to call out the city on Thursday during a public press conference, which was broadcast live on Facebook by WBNG.

“This is illegal — and it ends today,” he said.

“New York City is avoiding their responsibility to provide these people assistance and transferring their government responsibilities right here to Broome County taxpayers,” Garnar added. “One year rent is paid up front by New York City, then New York City abandons them and they now become the responsibility of Broome County.”

According to local officials, the NYC Human Resources Administration — which is responsible for the SOTA program — is in “direct contradiction” to New York state social services law 148, which is a criminal statute.

It states that “no person shall, without legal authority, send or bring, or cause to be sent or brought, any needy person into a public welfare district with the purpose of making him a charge on such public welfare district, or for the purpose of avoiding the responsibility of assistance or care in the public welfare district from which he is brought or sent.”

Such an act is considered a misdemeanor and liable to a fine.

“We realized this was occurring because one of our DSS supervisors was noticing a trend where recipients were coming in to DSS to apply for services and were reporting that they didn’t need help with their rental payments because their rent had been paid for for an entire year by the [HRA],” explained Nancy Williams, commissioner of the Department of Social Services.

“That caused our supervisor to question that because it didn’t seem right, it just didn’t seem within the law,” Williams said. “It was something she had not ever seen before.”

Thanks to the DSS worker’s diligence, the secret SOTA refugees were uncovered last week by county officials, who then brought the issue to the attention of their legal council. A thorough investigation was conducted and social services later confirmed that the families had come from a Big Apple homeless shelter as part of the SOTA program.

“Their rent had been prepaid — at an even higher rate than we normally would pay — to entice these homeless residents, these homeless folks from New York City to move up here,” Williams said. “So, although their rent has been paid for, they can receive other benefits from Broome County. They can receive SNAP benefits for their food, they can receive medical assistance and they can also receive utility support, to pay their utilities. So although New York City paid their rent for a year, we would be responsible for paying for other assistance programs.

“That could place a burden on Broome County,” Williams added. “It is our responsibility to take care of our residents and, as Jason mentioned, it is New York City’s responsibility to take care of their residents. This is really pushing the responsibility onto Broome County and it could really be taxing towards our resources, as well as our budget. If after a year, [the homeless residents] are still here, more than likely they will come back to DSS and they will need rental subsidy if they’re not employed. And if they’re eligible, we will need to then pay their rent after a year.”

To make up for the five families currently living in Broome — and “any other cases that may be identified” — local officials have asked the city to compensate them “for ongoing assistance costs incurred.”

They are also asking the de Blasio administration to immediately stop using the SOTA program or any other process to “illegally transfer its homeless people to Broome County.”

Officials believe they’ve been shipping homeless families off for months.

“We believe it’s happening throughout upstate New York,” explained Deputy Broome County attorney Howard Schultz, who is also chief attorney for the Department of Social Services.

“These families should have never been, basically excommunicated from New York City,” he said. “New York City has a homeless problem. Broome County and every county, to some degree has a homeless problem. [But] Broome County takes its homeless problem seriously, and not cynically like New York City does.

“In Broome County, we inspect our homeless shelters to make sure they are safe and secure for indigent homeless people,” Schultz said. “New York City, as reported, has serious problems in this area. There’s all kinds of crime that occurs in their homeless shelters and other undesirable outcomes for people in that shelter system in New York City…They don’t take [the issue] serious enough.”

Broome officials reached out to the de Blasio administration last week and made their demands, according to Schultz.

He said the city confirmed that five families were sent upstate, but wouldn’t comment on other cases.

City officials told The Post that they’ve been using relocation programs since as far back as the Koch administration — and that no one is being “illegally” transferred.

“Homeless families seeking housing where they can afford it and employment where they can find it should have access to those opportunities to get back on their feet,” said City Hall spokesman Isaac McGinn. “We’re helping families that have found housing transition out of shelter, as we’ve been doing with different programs for decades.”

The SOTA program was quietly launched last fall in part with “Project Reconnect,” which grants an all-expenses paid trip to anyone who’s been in the shelter system for at least 90 days and has a source of income. Over the past five years, the city has shelled out more than $1 million in travel expenses for 2,512 families — with each one relocating outside the five boroughs.

With additional reporting by Yoav Gonen