NEW YORK— Airline passengers have come to expect a tiny escape from the confined space of today’s packed planes: the ability to recline their seat a few inches. When one passenger was denied that bit of personal space Sunday, it led to a heated argument and the unscheduled landing of their plane, just halfway to its destination.

The fight started on a United Airlines flight because one passenger was using the Knee Defender, a $21.95 gadget that attaches to a passenger’s tray table and prevents the person in front of them from reclining.

The Federal Aviation Administration leaves it up to individual airlines to set rules about the device. United Airlines said it prohibits use of the device, like all major U.S. airlines. Spirit Airlines and Allegiant Air take the reclining mechanisms out of their seats, leaving them permanently upright.

The dispute on United Flight 1462 from Newark, New Jersey to Denver escalated to the point where the airline decided to divert to Chicago’s O’Hare International Airport, according to Transportation Security Administration spokesman Ross Feinstein.

Chicago Police and TSA officers met the flight, spoke to the passengers— a man and a woman, both 48— and “deemed it a customer service issue,” Feinstein said. The TSA would not name the passengers.

The plane then continued to Denver without them, arriving 1 hour and 38 minutes late, according to the airline’s website.

The Federal Aviation Administration can impose a civil fine of up to $25,000 for passengers who are unruly. In this case, no arrest was made, according to airport spokesman Gregg Cunningham.

The fight started when the male passenger, seated in a middle seat of row 12, used the Knee Defender to stop the woman in front of him from reclining while he was on his laptop, according to a law enforcement official with knowledge of the situation who spoke on condition of anonymity because they are not authorized to speak.

A flight attendant asked him to remove the device and he refused. The woman then stood up, turned around and threw a cup of water at him, the official says. That’s when United decided to land in Chicago. The two passengers were not allowed to continue to Denver.

Both passengers were sitting in United’s Economy Plus section, the part of the plane that has four more inches of legroom than the rest of coach.