United Airlines bars two girls from boarding the plane for wearing leggings

Another girl had to change before she was allowed to board the flight

United Airlines has barred two girls from boarding a flight because they were wearing leggings, sparking a firestorm of backlash online with users criticising what they called an intrusive and sexist policy.

The girls, whose ages were not specified, were not allowed onto the morning flight on Sunday from Denver to Minneapolis as they were travelling under an employee travel pass that includes a specific dress code, airline spokesman Jonathan Guerin said.

Another girl who was also wearing leggings had to change before she was allowed to board the flight.

The airline's dress code bars pass travellers from wearing spandex or Lycra pants such as leggings, but its actions sparked a quick backlash on Twitter, New York Times said.

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

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

"I guess @united not letting women wear athletic wear?" Watts tweeted to her more than 32,000 followers.

United Airlines responded by saying it "shall have the right to refuse passengers who are not properly clothed".

Angry netizens asked whether the airline was saying that it could deny service to paying customers just because they wore yoga pants.

"Leggings are the business attire for 10-year-olds. Their business is being children," tweeted actress Patricia Arquette.

"It's not going to be a pretty sight, but I'm going to wear yoga pants for my next @united flight," said another citizen.

Hours later, the airline said that the passengers who were flying "were not in compliance with our dress code policy for company benefit travel".

The longstanding policy of the airlines requires those who enjoy the perks of airline employment, which include free travel passes for family and guests, to present themselves in a way that represents the airline well.

According to the policy, "pass riders" aren't allowed to wear clothing that doesn't look "neat and professional".

That includes form-fitting lycra or spandex tops, pants and dresses, offensive or derogatory words or graphics on clothing, "excessively dirty" clothing that has holes or tears, or anything that is "inappropriately revealing."

United Airlines said that "leggings are not inappropriate attire except in the case of someone travelling as a pass rider."

But others weren't convinced the policy had merit.

"We here at @united are just trying to police the attire of the daughters of our employees!" read a sarcastic tweet from actor and comedian Seth Rogen. "That's all! Cool, right?"

United, a major American airline headquartered in Chicago, Illinois, is the world's third-largest when measured by revenue.