It was said to be made of soft white kid leather, given to him as a joke Christmas present by Princess Anne, his sister.
But the Prince of Wales found his personal lavatory seat so comfortable that it was packed, at his insistence, every time he travelled abroad. Or so the rumour went.
Now, the heir to the throne has had his own say, apparently so frustrated by the absurdity of the claim that he could not help but swear.
"My own what?" he asked, incredulously, when asked about the allegation during a radio interview in Australia. "Oh don't believe all that crap. The very idea."
The rumour was revived in a new biography, Rebel Prince: The Power, Passion and Defiance of Prince Charles, by Tom Bower. But it has been published and republished many times over the years and never, until now, been addressed by the heir to the throne.
He was caught off guard when asked about the mythical lavatory seat by a DJ from Brisbane's Hit105 station as he carried out an engagement in the city alongside the Duchess of Cornwall, Camilla, during their seven-day tour of Australia.
Camilla was also cornered and asked if her husband's denial was true. "So he doesn't carry his own toilet seat when he travels?" the reporter asked. "Don't you believe that," she laughed.
Julian Payne, the Prince's director of communications, later tweeted: "The Prince and the Duchess' tour of Australia and Vanuatu begins: 30 engagements, 7 days, 1 Commonwealth Games, 0 personal loo seats."
The seat is one of the Prince's many alleged habits and eccentricities for which he has long been lampooned.
It is often accompanied, as in the latest biography, by the claim that he also travels with his own personal supply of Kleenex Velvet lavatory paper, something yet to be confirmed or denied.
Other snippets in royal folklore include the claim that Charles has his toothpaste squeezed on to his toothbrush by his valet.
He has been said to ship his entire bedroom ahead of him when he stays with friends, and employs Indian military veterans to pick slugs off his plants by torchlight. He has also been said to take his own organic food on his travels and milk from the Windsor herd.
When the Prince turned 64 in 2012, Clarence House scotched many of these oft-repeated rumours on its website. It was not true, for example, that at breakfast he is presented with seven boiled eggs, each cooked to varying degrees of softness and lined up so he can choose his favourite.
He does, however, pay income tax and regularly attends church. No word yet on the Indian slug-pickers.