During the holiday season, United Airlines gives its flight attendants some leeway on how to accessorise their uniforms.

But the US airline based in Chicago apparently doesn't want its flight crews to go overboard with extras that detract from the flight attendants' professional image.

That was the key takeaway from a United Airlines memo sent to flight attendants released last week under the headline 'Holiday Adornment'.

"Accessories must be in good taste," the memo read.

"In the spirit of the holiday, and not detract from your professional image."

The memo outlines what "adornments" flight attendants are permitted to add to their uniforms, such as a "conservative holiday scarf" and "conservative earrings" for women and "conservative holiday tie" and "one holiday pin" for men.

But the memo also makes clear what is prohibited: "Head adornments (i.e., antlers, Santa hats, haloes, etc.; Holiday vests or sweaters; Holiday aprons: Holiday hosiery."

A United Airlines spokeswoman said the memo is issued every year at this time to remind employees of the carrier's standards but she said she didn't know how many years it has been sent out and what prompted the original memo.