Two girls were turned back from boarding a flight because they were wearing leggings, passengers have reported.
US airline United has faced anger after its agent apparently ordered girls wearing the sportswear to change - or facing losing their seats if they did not have alternative clothing.
The airline appears to have confirmed the incident with a post on its Twitter account saying they reserve the right to turn away passengers if they are "not properly clothed".
It later added it was investigating the incident.
Passenger Shannon Watts reported that three girls were told they would not be allowed onto a Denver to Minneapolis flight on Sunday because they were wearing leggings.
One was later allowed to board after putting on a dress, while two others who did not have extra clothing were seemingly refused entry to the plane.
United's Twitter account responded to angry posts over the incident with a tweet confirming that the airline reserves the right to turn away people in they are not deemed to be properly dressed.
The only mention of clothing in the airline's terms and conditions appears to be in the section over safety, saying that passengers must not be “barefoot or not properly clothed”.
United has faced disbelief and criticism, with many saying it was baffling that the sports clothing would be banned.
United later said its customer service team were "currently looking into the situation."
A spokesman was not immediately available for comment.