EDINBURGH, Scotland—  A true Scotsman is said to never wear anything under his kilt. But now Scots are being warned that the sartorial tradition could be both indecent and unhygienic.

The organization charged with maintaining standards in Highland dress has backed kilt-rental firms who say the age-old custom of “going commando” has got to go.

“We are saying please use common sense and decency as it can be unhygienic and it can be offensive,” Brian Wilton of the Scottish Tartans Authority said Tuesday.

“If you are hiring a kilt, then wear underwear because some of them are left in a horrible state.”

The tradition comes from the clansmen warriors of the Highlands who wore little but long kilts that wrapped around the shoulder. The Scottish military took up the idea, and Scots regiments still parade sans underwear to this day.

“My father was a sergeant major in the army and used to clip a car mirror on the end of his pace stick and held it under the kilts of his men to check they were suitably undressed for parades,” Wilton said.

Wilton also offered etiquette advice for anyone meeting a man in a kilt: “Really you shouldn’t ask a Scotsman what he wears under his kilt.” he said. “Imagine the offense you would cause if you were to ask the same of a woman” Howie Nicholsby, of innovative tailoring firm 21st Century Kilts in Edinburgh, has worn only kilts for the last 14 years and does not own trousers. He agrees that underwear is often an asset. “In my line of work I am very active and the capacity for wardrobe malfunction is endless,” he said.