United Airlines has come under fire on social media after two girls were reportedly barred from flying for wearing leggings.

The incident happened on a flight from Denver to Minneapolis on March 26, activist Shannon Watts said.

United said the girls were travelling on a special pass, for employees and their guests, which has a dress code.

The airliner has since clarified that all regular, paying customers are welcome to wear leggings.

The girls were “United pass travelers”, which are tickets for company employees or eligible dependents, it explained in a Twitter exchange on the issue.

“United pass travelers” are eligible for free or heavily discounted air travel.

The dress code for pass-holders states they should not wear “form-fitting lycra/spandex tops, pants and dresses”, among various other stipulations, including no exposed midriffs, no mini skirts and no flip-flops.

Shannon Watts tweeted about what happened to five girls when they tried to board a flight at Denver airport.

The activist said a United gate agent was “forcing” the girls, one of them aged 10, to change their clothes or wear dresses over the leggings.

Shannon Wats said three of the girls were allowed to fly after putting dresses over the top of their clothing, but two were prevented from boarding.

She criticized the airliner for its actions, asking: “Since when does United police women’s clothing?”

United later released a statement explaining the dress code requirement of its United pass travelers.

The statement 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.”

It added that the passengers on that flight had not complied to the dress code.

Shannon Watts’ tweets have been shared and responded to by thousands of users, including Patricia Arquette.

Flight attendant and author Heather Poole, who does not work for United, defended the airline’s policy on Twitter, saying the dress codes for staff travelling on free passes are common practice in the industry, and have evolved over the years.