The contract's only mention of dress code is to allow agents the right to refuse anyone who is 'barefoot or not properly clothed'.
Stunned critics lambasted the airline for defending the policy.
'If leggings aren't allowed then what about shorts and dresses. Sounds like united Airlines is bodyshaming,' said one.
'United Airlines is defending their decision to prevent pre-teen girls wearing leggings from boarding a flight due to their attire,' another wrote alongside a string of thumbs-down emojis.
Its Twitter operators continued to defend the decision without going in to further detail on Sunday morning.