A gate agent for a United Airlines flight temporarily barred two girls from getting on a flight Sunday morning because they were wearing leggings, a witness tweeted.

Twitter user Shannon Watts, who was on the same flight as the girls, reported that they were asked to change or “put dresses on over” their leggings before boarding the flight from Denver to Minneapolis.

Watts described the incident in a series of tweets.

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"A @united gate agent isn't letting girls in leggings get on flight from Denver to Minneapolis because spandex is not allowed?” Watts tweeted.

"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?”

Watts tweeted that the girls were finally allowed to board after putting dresses on top of what they were wearing.

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

3) 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? — Shannon Watts (@shannonrwatts) March 26, 2017

The United Airlines Twitter account responded to tweets complaining about the incident on social media said that "United shall have the right to refuse passengers who are not properly clothed via our Contract of Carriage."

@baddestmamajama United shall have the right to refuse passengers who are not properly clothed via our Contract of Carriage. ^FS — United (@united) March 26, 2017

In a subsequent tweet, United said the passengers were using an employee benefit pass and "were not in compliance with our dress code policy for company benefit travel."

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

United specifies in its passenger contract that the airline can refuse a passenger if the passenger is “barefoot or not properly clothed."

United Airlines spokesman Jonathan Guerin later told The Washington Post that there's a specific policy for those using the travel benefit that forbids leggings.

“Our regular passengers are not going to be denied boarding because they are wearing leggings or yoga pants,” Guerin told the Post. “But when flying as a pass traveler, we require these pass travelers to follow rules, and that is one of those rules."

United did not immediately respond to a request from The Hill for comment.

Per @united, this is the line in their passenger contract that allows them to refuse service to women wearing leggings pic.twitter.com/poIG4HgqL7 — Christopher Ingraham (@_cingraham) March 26, 2017

.@united They just boarded after being forced to change or put dresses on over the top of their clothing. Is this your policy?

— Shannon Watts (@shannonrwatts) March 26, 2017