United Airlines faced a fierce backlash online after a gate agent barred two teenage girls from boarding a flight from Denver to Minneapolis because they were wearing leggings. Shannon Watts, founder of the anti-gun violence group Moms Demand Action, first highlighted the incident in a series of tweets saying she saw a gate agent force the girls to change or wear dresses over the leggings or they would not be permitted to board. Watts described one girl who was not permitted to board the flight as a 10-year-old wearing grey leggings.

"Since when does @united police women's clothing?" Watts tweeted. "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?" "This behaviour is sexist and sexualises young girls," she continued. "Not to mention that the families were mortified and incovenienced."

In response, United tweeted: "In our Contract of Carriage, Rule 21, we do have the right to refuse transport for passengers who are "barefoot or not properly clothed", and noted that the girls were pass riders and not regular airline customers.

"There is a dress code for pass travellers as they are representing UA when they fly," the airlines said.

In response to one Twitter user who pointed out that its "properly clothed" is not defined, United said: "This is left to the discretion of the gate agents."

The airlines later released a statement clarifying their position on the issue and said regular customers are still permitted to wear leggings on their flights. United explained that "pass riders" - relatives or friends of United employees - receive free or discounted air travel on United flights as well as other partner airlines.

"We care about the way we present themselves to you, our customers, as we believe that is part of the experience on board our flights," United said. "When taking advantage of this benefit, all employees and pass riders are considered representatives of United. And like most companies, we have a dress code that we ask employees and pass riders to follow.

"The passengers this morning were United pass riders and not in compliance with our dress code for company benefit travel. We regularly remind our employees that when they place a family member or friend on a flight for free as a standby passenger, they need to follow our dress code.

"To our regular customers, your leggings are welcome."

The incident, however, quickly went viral on social media triggering fierce criticism, particularly from celebrities, who slammed the airline's policies as "sexist" and "outdated" using the trending hashtag #LeggingsGate.

While some Twitter users pointed out evidence of United using leggings, yoga pants and other attire in their promotional material, others responded with a slew of hilarious memes and jokes in response to United's policy. Some users also questioned the outrage saying people travelling on a discounted employee pass should adhere by the company's dress codes.