Border Patrol agents in upstate New York have been redeployed to deal with the migrant crisis on the southern border — leaving large sections of the northern border unguarded, The Post has learned.

“There’s a good portion of our patrol on the Mexican border. There are times when swaths of the northern border with Canada are not patrolled,” a source inside the northern Border Patrol said.

The insider said at least 15 percent of patrol staff were sent south.

Because discussing deployment could be considered a national security breach, the source declined to give his name for fear of retaliation from Homeland Security brass.

“We were handicapped before the latest crisis at the southern border. Now you’re taking away the wheelchair,” the source said.

“I definitely see this as being a security issue.”

A second northern Border Patrol agent said, “I’ve never seen the manpower shortage along the Canadian border as low as it is now. It’s low across the board.”

Upstate Rep. Elise Stefanik, whose district includes many communities along the Canadian border, said the drop in manpower has emerged as a serious concern.

“Over the past several weeks, I have heard from many whistleblowers from the US Border Patrol expressing grave concerns over the lack of manpower along our Northern Border, as agents are being transferred to clean up President Biden’s crisis in the south,” Stefanik said.

Stefanik said even Border Patrol agents who remain in New York are actually working remotely on the crisis at the southern border.

“The few agents that remain are forced to virtually process illegal immigrants in Texas, leaving only a handful to patrol the entire New York Border and zero to patrol our waterways,” she said.

“President Biden’s immigration, economic, humanitarian, and national security crisis at the US-Mexico border has in turn created a crisis at the US-Canada border. He must put an end to this madness, protect the American people, and prioritize the well-being of our Border Patrol Agents.”

A Border Patrol source said boat patrols along the St. Lawrence River used to catch many illegal immigrants on the northern border have been reduced as resources shifted south.

The Department of Homeland Security’s Customs and Border Protection unit confirmed that hundreds of its field agents have been reassigned to the southern border to deal with the surge in migrants.

“Due to fluctuations along the Southwest Border, U.S. Customs and Border Protection has deployed more than 300 Border Patrol agents primarily from northern and coastal sectors to support operations. CBP seeks to deter and disrupt human smuggling activities by transnational criminal organizations and ensure our personnel are properly equipped to maintain border security,” said CBP spokeswoman Stephanie Malin.

Citing security concerns, she declined further comment on total deployment numbers, locations and other details.

The number of people apprehended for illegally crossing from Canada into the United States along its northern border nearly tripled from 2017 to 2020 and a growing portion were Mexican citizens, according to federal data released last year.

Under current Canadian law, Mexicans do not require a visa to enter the country. So many Mexicans arrange a flight to Toronto or Montreal and then cross the New York border.

In the past year, there also has been a 1,000 percent increase in drug seizures along the northern border and a 30 percent increase in illegal firearm seizures, Stefanik said.