If there is one place where you certainly shouldn’t have to worry about looking good or being judged, it’s on an airplane.

Everyone who has ever travelled knows that when it comes to air travel, fashion, beauty and comfort do not mix.

But it turns out that there are some rules about what we can and can’t wear on a plane that we never knew about.

Apparently, leggings are banned.

Leggings like these? NOT ALLOWED. Image via iStock.

The air-travelling world is in shock after one woman live-tweeted a 'showdown' of sorts at an airline gate, after two girls were told they had to change out of their comfortable activewear before they were allowed to board the plane.

Yes, really.

Passenger Shannon Watts - who started Moms Demand Action, a group campaigning for reasonable solutions to address our nation’s culture of gun violence - shared the story with her 33,000 followers.

1) A @united gate agent isn't letting girls in leggings get on flight from Denver to Minneapolis because spandex is not allowed?

— Shannon Watts (@shannonrwatts) March 26, 2017

2) She's forcing them to change or put dresses on over leggings or they can't board. Since when does @united police women's clothing?

— Shannon Watts (@shannonrwatts) March 26, 2017

3) Gate agent for flt 215 at 7:55. Said she doesn't make the rules, just follows them. I guess @united not letting women wear athletic wear?

— Shannon Watts (@shannonrwatts) March 26, 2017

United Airlines responded to Shannon's series of tweets, stating that they "have the right to refuse transport for passengers who are barefoot or not properly clothed".

We all know leggings probably aren't the most suitable attire for, say, walking the red carpet at the Oscars (although, each to their own, right?), but to label the pants as "inappropriate" to wear on a flight is a bit of a stretch.

Especially when one of the passengers in question was understood to be just 10 years old.

Shannon argued that forcing the girls to change - or in one of the cases, put a dress over her leggings - was not only humiliating, but actually sexualised the young passengers.

2) This behavior is sexist and sexualizes young girls. Not to mention that the families were mortified and inconveninced.

— Shannon Watts (@shannonrwatts) March 26, 2017

United said that the passengers in question were actually "pass riders", which means they were United employees or their eligible dependents.

According to the airline, there are "separate dress codes" for pass riders, that remains "internal policy" and cannot be released.

The airline has now been hit with backlash from many of their frequent flyers, including some famous names.

I have flown united before with literally no pants on. Just a top as a dress. Next time I will wear only jeans and a scarf.

— christine teigen (@chrissyteigen) March 26, 2017

Turns out United bans leggings, torn jeans, and shorter shorts for free employee pass tickets. Still @united should update policy.

— Patricia Arquette (@PattyArquette) March 26, 2017

Hey @united I fly a LOT. About to go on tour all April and changing all my @united flights to other airlines

— Sarah Silverman (@SarahKSilverman) March 26, 2017

????????????I'm going to start wearing leggings! Is that against the rules? ???? #outdatedfashion #outdatedrules https://t.co/V5LnwDwygY

— William Shatner (@WilliamShatner) March 26, 2017

We here at @united are just trying to police the attire of the daughters of our employees! That's all! Cool, right? https://t.co/xGyL4IAslE

— Seth Rogen (@Sethrogen) March 26, 2017

Others simply pointed out that United's own marketing campaigns have shown women wearing leggings boarding planes. Or, you know, they just made memes:

ATTENTION TRAVELERS here is United Airlines new dress code for women. Cp pic.twitter.com/ReZP6F90Uz

— Philanthropit ???????? (@RajbarSab) March 26, 2017

Here's an idea,

Everyone flying on United tomorrow should wear leggings. Let's see what happens. @united

— Jared Montana (@Jarmadillos) March 26, 2017

@united Really? pic.twitter.com/O9RplgrTsv

— Jeremy Pond (@JeremyPond) March 26, 2017