United Airlines has been heavily criticised on social media after two girls were reportedly barred from flying for wearing leggings.
The incident happened on a flight from Denver to Minneapolis, in the US, on Sunday morning, activist Shannon Watts said.
A United gate agent was "forcing" the girls, one of them aged 10, to change their clothes or wear dresses over the leggings, she tweeted.
United said the girls were travelling on a ticket that had a dress code.
Shannon Watts, founder of the group Moms Demand Action for gun reforms, tweeted about what happened to five girls when they tried to board a flight at Denver airport.
She said three of the girls were allowed to fly after putting dresses over the top of their clothing, but two were prevented from boarding.
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
She said the behaviour was sexist and slammed the airliner for its actions, asking: "Since when does United police women's clothing?".
Although United has not officially commented on the incident, it did responded on Twitter by explaining the dress code requirement of its United pass travellers.
The passengers this morning were United pass riders who were not in compliance with our dress code policy for company benefit travel.— United (@united) March 26, 2017
Ms Watts' tweets have been shared and responded to by thousands of users, including the actress and activist Patricia Arquette.
@united Leggings are business attire for 10 year olds. Their business is being children.— Patricia Arquette (@PattyArquette) March 26, 2017