United Airlines has responded to a set of viral tweets that allege the company did not allow young girls wearing leggings to board a flight Sunday morning.

Shannon Watts tweeted that the young passengers were forced to change clothes before boarding a flight from Denver to Minneapolis. Watts told 9NEWS she is not associated with the family and was boarding a different flight to Mexico.

United says the passengers not allowed to board were pass riders, who fly under employee pass privileges. Pass riders are defined as employees or people related to United employees.

According to Jonathan Guerin, a United Airlines spokesperson, yoga pants or leggings are more than welcome for regular paying customers.

Pass riders have a stricter dress code to board. The three people involved in the incident did not meet the criteria for pass riders.

Pass riders are considered representatives of United and that extends to the dress code requirements. Casual attire for pass riders is allowed as long as it is in good taste for the environment.

As an example, Guerin says flip flops are not allowed for pass riders.

9NEWS asked to see the dress code policy for pass riders. Guerin says the information is part of an internal policy and will not be released at this time.

If the passengers were flying as regular customers, they would have been allowed on board.

Guerin says the three people involved in the incident stayed together and will be on the next flight, assuming they meet dress code.