A flight between New Jersey and Denver had to be diverted to Chicago after two passengers began fighting over a controversial 20 dollar device which prevents plane seats reclining.

The mid-air argument on United Airlines Flight 1462 began when a man in the Economy Plus section attached a "Knee Defender" to the arms of his tray table so he could work on his laptop.

The woman in front apparently complained to a steward who asked him to move the device - banned from all major US airlines - but the man refused. The woman then stood, turned and threw a cup of water over him. An altercation between the two 48-year-olds ensued and the decision was made to land at Chicago O'Hare airport. Police were waiting as the plane landed.

If found guilty of unruly behaviour both passengers could have been fined $25,000 but authorities deemed the incident "a customer service issue", said Transportation Security Administration spokesman Ross Feinstein.

Neither passenger was allowed to rejoin the flight which then departed for Denver, arriving one hour 38 minutes late. It is not known how the two passengers made the 1,000 mile trip from Chicago to Denver, or if they opted to travel on together.

The New Jersey-Denver flight had to be diverted to Chicago due to the altercation Reuters

Although the US Federal Aviation Administration says it is up to individual airlines to decide whether allow the $21.95 device (tagline: 'Standing up for the right of the tall guy to sit down'), most airlines - including United - have banned it.

Inventor of the Knee Defender Ira Goldman, 6'2", said passengers using the device should inform the person sitting in front if they plan to use them. The gadget even comes with a little courtesy card which can be given to the unlucky person in front which reads: "I realize that this may be an inconvenience. If so, I hope you will complain to the airline. Maybe working together we can convince the airlines to provide enough space between rows so that people can recline their seats without banging into other passengers."

Knee Defender Goldman told USA Today: "The Knee Defender says right on it: 'Be courteous. Do not hog space. Listen to the flight crew.' Apparently that is not what happened here."