A Westchester mom said United refused to allow her to board a flight with a carry-on bag full of medical supplies for her chronically ill 22-year-old daughter — and even warned her not to “mess” with the flight crew.

Jill Levy-Fisch of Tarrytown said she and her daughter Sara, who suffers from a host of medical problems, were scheduled to fly from Newark to Halifax in Canada for vacation on June 28.

“Sara suffers from a severe illness that requires constant access to her medications, many administered through her chest port that must be temperature controlled,” she told The Post.

“The bag has a tag clearly indicating medical equipment,” said Levy-Fisch, 54, adding that her family has flown the route often with United, whose flight attendants have always allowed her to stow the bag in a closet.

Levy-Fisch said she had called the United disabilities line and was assured that the medical equipment could not be banned from the cabin.

But she said a gate agent insisted that the bag be checked in for the baggage hold — telling her that she had to remove any medications she needed from the bag if she wanted to bring them into the cabin.

“We told him he’d be checking an empty bag because it only contained medications. We asked to speak with a gate supervisor and he claimed to be one himself,” she said. “He was belligerent, abusive and disrespectful. We asked to speak with a flight attendant.”

The distraught woman said that after the agent accompanied her to the regional jet’s door, berating her along the way, a flight attendant said: “You do not want to mess with this crew.”

“We were shocked, upset and brought to tears. He said we couldn’t bring the medical bag on the plane,” Levy-Fisch said, adding that she had to board without her daughter because some medications were already in Canada and she needed to pick them up from where they were being stored.

Among other ailments, Sara suffers from autoimmune encephalitis, Lyme disease and postural orthostatic tachycardia syndrome, or POTS, a condition in which too little blood returns to the heart while standing up, Levy-Fisch said.

Sara’s carry-on bag contained 30 vials of IV medications, an IV pump, bags of saline, various medications, syringes, a glucose monitor and a sleep apnea device, the mom said.

“She left in tears. This was to be a fun family vacation and she didn’t deserve the treatment she received,” Levy-Fisch said of Sara, who ended up taking a Delta flight the next day.

“Once I boarded the plane, I sat quietly crying,” she added. “The flight attendant approached me to say the pilot wanted to know if I planned to ‘behave’ on the flight.

“I was afraid to say anything as I needed to get the medications at our destination and was concerned they would remove me from the plane.”

Levy-Fisch said she messaged United through Facebook prior to takeoff and was asked to get the flight attendant’s name — no easy task, she said, because he apparently hid his name badge.

Eventually, he relented and gave her his first name — Carlos, she said.

She said a United rep called her after the nightmarish flight and said she would try to have the gate agent reprimanded — but that it would be tougher to take action against the heartless crew member because he worked for a regional subsidiary.

Levy-Fisch, president of the Save Babies Through Screening Foundation, said the airline refunded her daughter’s ticket, but she wanted “to ensure that such unprofessional, demeaning behavior never happens again to another person traveling on United with a disability.”

United spokeswoman Maddie King blamed the debacle on the size of the regional aircraft, which she said has smaller overhead bins that can’t fit larger bags.

“Because of this, there was no space onboard the aircraft to stow this customer’s bag, and it would only fit checked underneath the aircraft,” King told The Post.

But Levy-Fisch said she and her daughter were the first to board, “so they can’t say there wasn’t room in the closet.”

King added: “We reached out to apologize to our customer for this experience, and we refunded her ticket for the flight as a gesture of goodwill,” she said.

“We’ve also reached out to our partners at Express Jet, who operated the flight, and our team in Newark to review the way this situation was handled.”

In a statement, Express Jet rep Jarek Beem said: “We have also reviewed this flight with our crew members and apologize for any disruptions to our customers’ travel plans.”