United Airlines came under fire Sunday after two teenage girls were barred by a gate attendant from boarding a flight from Denver to Minneapolis because they were wearing leggings.

The girls, whose ages were not specified, were not allowed onto the morning flight because they were traveling under an employee travel pass that includes a specific dress code, airline spokesman Jonathan Guerin said.

The dress code bars pass travelers from wearing spandex or Lycra pants such as leggings. The teenagers agreed to change their clothing and take a later flight, Guerin said, but the airline's actions sparked a quick backlash on Twitter.

Activist Shannon Watts of Denver tweeted that she witnessed Sunday's events and questioned United's decision to police women's clothing.

Watts said the girl's father was allowed to board while wearing shorts and called the airline's policy sexist.

Regularly ticketed passengers are not subject to the same dress code and can wear leggings, Guerin said.

The airline doubled down on its dress code policy in a statement later Sunday, according to Fox 31 Denver.

“When taking advantage of this benefit, all employees and pass riders are considered representatives of United,” the airline 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. 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.”

The airline added, “to our regular customers, your leggings are welcome.”

The Associated Press contributed to this report.