WASHINGTON - United Airlines has run into a social media storm after it barred two teenage girls from boarding a flight in Denver because they were wearing leggings.

Another girl who was also wearing leggings was allowed to board the flight from Denver International Airport to Minneapolis after she changed, a witness said.

A United Airlines spokesman Jonathan Guerin told The New York Times that the two girls barred from flying "made an adjustment" to their clothing and were waiting for the next flight to Minneapolis. He did not know whether they made it aboard.

The incident was reported on Twitter by Shannon Watts - founder of gun reform group Moms Demand Action - who was a passenger at the airport waiting to board another flight to Mexico.

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

"A 10-year-old girl in gray leggings," she added of one of the girls. "She looked normal and appropriate."

With that, the charges began to fly across the Twittersphere, with outraged travelers and celebrities weighing in with accusations of sexism and corporate ham-handedness.

"I have flown united before with literally no pants on. Just a top as a dress," said model Chrissy Teigen in one tweet. "Next time I will wear only jeans and a scarf."

United Airlines defended the gate agent's decision in a series of tweets.

It insisted the girls were flying on passes that require them to keep to a dress code in return for free or discounted travel.

"When taking advantage of this benefit, all employees and pass riders are considered representatives of United," it said.

"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."

That only drew attention to what many saw as a double standard. Some wondered what business United had in deciding for others what was appropriate casual wear.

The leggings rule, tweeted John Balano, was "something I'd expect from the Taliban, not a major western airline".

"Leggings are business attire for 10-year-olds. Their business is being children," Academy Award-winning actress Patricia Arquette said.

Some men mocked United by threatening to wear leggings they next time they fly.

Actor William Shatner posted a picture of himself as younger man, bare chested and in red tights.

"See I've done it before," he said.