As one of the world’s biggest airlines tries to wriggle its way out of a row about passengers’ clothing, there are calls for the strict dress code for airline travellers with free tickets to be eased.

Shannon Watts, a passenger flying on United from Denver airport on Sunday, noticed that some young travellers at a nearby gate were being told to change. She tweeted: "@united gate agent isn't letting girls in leggings get on flight from Denver to Minneapolis because spandex is not allowed?"

The airline tweeted in response: “United shall have the right to refuse passengers who are not properly clothed via our Contract of Carriage”.

These rules say only that the airline will refuse to fly “Passengers who are barefoot or not properly clothed."

It later transpired that the passengers were travelling on free tickets, for which a strict dress code applies.

Most airlines stipulate higher standards of dress for staff and their families travelling on “non-revenue” tickets or passes.

United said: “We have a dress code that we ask employees and pass riders to follow. The passengers this morning were United pass riders and not in compliance with our dress code for company benefit travel.”

The rules stipulate:“Pass riders’ overall appearance should be well-groomed, neat, clean and in good taste.”

There are specific regulations about shorts (“no more than three inches above the knee when in a standing position”) and, crucially, “Form-fitting lycra/spandex tops, pants and dresses.”

But Ben Schlappig, author of the One Mile At A Time frequent-flyer blog, called the clothing guidelines “outdated”.

“The intent of course is that flying is a privilege for employees and their family, and they should have a nice appearance. Is that still necessary nowadays?”

Gary Leff, author of the View from the Wing blog, said: “It may be time to revisit non-rev dress code rules, norms change.”

He also criticised United for implying, during the Twitter altercation over the episode, that normal passengers could be barred for wearing leggings: “United still said that it would be acceptable for a gate agent to deny travel to revenue passengers wearing leggings and needs to walk that back.”

The airline duly did so, saying: “To our regular customers, your leggings are welcome.”

The Independent has identified similar rules among UK airlines.

British Airways instructs former staff travelling on free or discounted tickets: “Both yourself and your nominee/s should be neatly dressed, especially if your ticket offers premium cabin eligibility. For travel in the Club and Traveller cabins smart jeans can be worn.”

BA also says that non-revenue passengers must not accept amenity kits: “When travelling in a premium cabin wash bags and sleepers suits must be discreetly declined”.

The rules are also specific about drinks and meals: “When requesting a wine choice that has not been opened for a commercial passenger you are expected to discreetly accept an alternative choice.

“Additionally, our commercial customers will be given first choice from the food menu. If your menu choice is not available, discreetly accept an alternative meal.”

Virgin Atlantic is believed to have similar rules for staff and their families, which include bans on sarong, kaftans and shorts, skirts and dresses above mid-thigh.

Yet in a recent blog, the Virgin Atlantic founder, Sir Richard Branson, said: “Personally, I think all employers should drop the dress code and let their teams wear whatever they feel most comfortable in.”

A former executive for a UK airline said: "The staff travel dress code has become much more lenient over the past 20 years, but in the 80s staff passengers were expected to wear jackets and tie, even in economy. Jeans were forbidden in all classes.