King Charles III and Prince William's ex-butler has revealed they roasted him over his fake tan.

The former royal staffer revealed he was compared to an "Oompa Loompa" and made to show Prince William his toes.

Grant Harrold, 47, worked as a butler for both Charles and his two sons, William and Prince Harry, and shared the anecdote in an interview to promote his new memoir, The Royal Butler.

He explained the incident took place when he was working at Charles and Queen Camilla's family residence, Highgrove House.

Having driven to nearby Bath to get a spray tan, Grant was serving dinner to Charles, 76 and Camilla, 78, that evening when the couple noticed a strange smell in the air and asked him what it was. After he told them it was his new fake tan, they found it hilarious.

"Camilla was looking at me, just staring," Grant told People Magazine. "(Charles) said, 'What have you done? What colour did you pick, Oompa Loompa?'".

Later, when Prince William and Catherine, Princess of Wales - at the time, William's girlfriend - caught sight of the new look, they were equally amused.

"William said, 'What have you done? Has it gone everywhere?' and being told yes, he repeated, 'Everywhere?' I said it was 'everywhere.' He said, 'Can I see it?'" Grant recalled.

"I had to take my shoes and socks off to show him my toes, which were orange! He thought it was brilliant."