A United Airlines agent stopped two girls from flying because they were wearing leggings, Twitter user and activist Shannon Watts said on Sunday, sparking a firestorm of backlash online for policing women’s clothing. Responding to the criticism, airline officials said that they can “refuse passengers who are not properly clothed.”

Another girl wearing grey leggings had to change before she was allowed to board her flight at Denver International Airport. The witness said the girl appeared to be 10 years old and was attempting to fly from Denver to Minneapolis.

“She’s forcing them to change or put dresses on over leggings or they can’t board,” she tweeted. “Since when does @united police women’s clothing?”

At first, United did not issue a formal response to the tweets but said on Twitter that the company “shall have the right to refuse passengers who are not properly clothed via our Contract of Carriage.” They added, “This is left to the discretion of the agents.”

A United spokesperson told the Washington Post that the two teens were not allowed on the flight. They explained the girls were travelling with a United employee pass and “were not in compliance with our dress code policy for company benefit travel”.

Thousands attend Women's March on Washington Show all 18 1 /18 Thousands attend Women's March on Washington Thousands attend Women's March on Washington Protesters gather outside the White House at the finish of the Women's March on Washington on January 21, 2017 in Washington, DC. Large crowds attended the anti-Trump rally a day after U.S. President Donald Trump was sworn in as the 45th U.S. president. Getty Thousands attend Women's March on Washington Protesters hold up signage near the Washington Monument during the Women's March on Washington January 21, 2017 in Washington, DC. Large crowds are attending the anti-Trump rally a day after U.S. President Donald Trump was sworn in as the 45th U.S. president. Drew Angerer/Getty Thousands attend Women's March on Washington Protesters gather during the Women's March on Washington January 21, 2017 in Washington, DC. The march is expected to draw thousands from across the country to protest newly inaugurated President Donald Trump. Aaron P. Bernstein/Getty Thousands attend Women's March on Washington Protesters arrive at the Capital South Metro station for the Women's March on Washington on January 21, 2017 in Washington, DC. Following the inauguration of Donald Trump as the 45th president of the United States, the Women's March has spread to be a global march calling on all concerned citizens to stand up for equality, diversity and inclusion and for women's rights to be recognised around the world as human rights. Jessica Kourkounis/Getty Thousands attend Women's March on Washington Protesters arrive on the platform at the Capital South Metro station for the Women's March on Washington on January 21, 2017 in Washington, DC. Following the inauguration of Donald Trump as the 45th president of the United States, the Women's March has spread to be a global march calling on all concerned citizens to stand up for equality, diversity and inclusion and for women's rights to be recognised around the world as human rights. Jessica Kourkounis/Getty Thousands attend Women's March on Washington Demonstrators protest during the Women's March along Pennsylvania Avenue January 21, 2017 in Washington, DC. Hundreds of thousands of protesters spearheaded by women's rights groups demonstrated across the US to send a defiant message to US President Donald Trump. Joshua Lott/AFP/Getty Images Thousands attend Women's March on Washington WASHINGTON, DC - JANUARY 21: Protesters attend the Women's March on Washington on January 21, 2017 in Washington, DC. Following the inauguration of Donald Trump as the 45th president of the United States, the Women's March has spread to be a global march calling on all concerned citizens to stand up for equality, diversity and inclusion and for women's rights to be recognised around the world as human rights. Jessica Kourkounis/Getty Thousands attend Women's March on Washington A marcher holds a sign during the Women's March on Washington January 21, 2017 in Washington, DC. The march is expected to draw thousands from across the country to protest newly inaugurated President Donald Trump. Aaron P. Bernstein/Getty Thousands attend Women's March on Washington A woman chants while attending the Women's March on Washington on January 21, 2017 in Washington, DC. Large crowds are attending the anti-Trump rally a day after U.S. President Donald Trump was sworn in as the 45th U.S. president. Mario Tama/Getty Thousands attend Women's March on Washington Protesters attend the Women's March on Washington on January 21, 2017 in Washington, DC. Large crowds are attending the anti-Trump rally a day after U.S. President Donald Trump was sworn in as the 45th U.S. president. Mario Tama/Getty Thousands attend Women's March on Washington Protesters march in Washington, DC, during the Women's March on January 21, 2017. Hundreds of thousands of people flooded US cities Saturday in a day of women's rights protests to mark President Donald Trump's first full day in office. Robyn Beck/AFP/Getty Images Thousands attend Women's March on Washington A protester gestures toward the White House on the Ellipse near the South Lawn of the White House during the Women's March on Washington January 21, 2017 in Washington, DC. Large crowds are attending the anti-Trump rally a day after U.S. President Donald Trump was sworn in as the 45th U.S. president. Drew Angerer/Getty Thousands attend Women's March on Washington A protester, holding a Donald Trump doll wearing a pink cap, marches in Washington, DC, during the Womens March on January 21, 2017. Hundreds of thousands of people flooded US cities Saturday in a day of women's rights protests to mark President Donald Trump's first full day in office. Robyn Beck/AFP/Getty Images Thousands attend Women's March on Washington Protesters take to the National Mall to demonstrate against the presidency of Donald Trump Washington, DC on January 21, 2017. Hundreds of thousands of protesters spearheaded by women's rights groups demonstrated across the US to send a defiant message to US President Donald Trump. Dominick Reuter/AFP/Getty Images Thousands attend Women's March on Washington Protesters march during the Women's March on Washington on January 21, 2017 in Washington, DC. Large crowds are attending the anti-Trump rally a day after U.S. President Donald Trump was sworn in as the 45th U.S. president. Mario Tama/Getty Thousands attend Women's March on Washington Demonstrators gather on The Ellipse during the Women's March on Washington January 21, 2017 in Washington, DC. Hundreds of thousands of protesters spearheaded by women's rights groups demonstrated across the US to send a defiant message to US President Donald Trump. Zach Gibson/AFP/Getty Images Thousands attend Women's March on Washington Demonstrators march down Pennsylvania Avenue during the Women's March on Washington January 21, 2017 in Washington, DC. Hundreds of thousands of protesters spearheaded by women's rights groups demonstrated across the US to send a defiant message to US President Donald Trump. Zach Gibson/AFP/Getty Images Thousands attend Women's March on Washington Protester's signs are left near the White House during the Women's March on Washington on January 21, 2017 in Washington, DC. Large crowds are attending the anti-Trump rally a day after U.S. President Donald Trump was sworn in as the 45th U.S. president. (Photo by Mario Tama/Getty Images) Mario Tama/Getty

Still, many people online expressed outrage over the incident, suggesting many women wear leggings and yoga pants for comfort while travelling.