Prince Charles and his wife Camilla reportedly wrote naughty jokes on a chocolate box which is now set to go to auction.

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The royal couple – who married in 2005 – wrote on the tin while staying at Blenheim Palace, Oxfordshire for Christmas in the late 70s, and it is being cited as early evidence of the long friendship the pair have enjoyed.

The Fortnum and Mason chocolate box depicts Mr Fortnum and Mr Mason delivering Christmas presents in a horse drawn carriage.

Camilla and Charles have drawn a pile of manure and a puddle under the horse, and written jokes about diarrhoea and yellow rivers.

They use what would nowadays be considered homophobic comments on the gents riding in the carriage, calling the pair “two old p**fs” and the gifts in the picture “p**f presents”. The store logo is renamed “fresh and messy”.

The tin includes the two royals’ signatures and is expected to fetch around £600 at auction in Ludlow, Shropshire.

Richard Westwood-Brookes, of Mullock’s Auctioneers, said: “This is clearly the product of a jovial evening between Charles and Camilla. Some of the comments are somewhat ribald.”

The chocolate box was given to a member of staff at Blenheim Palace, and Charles and Camilla’s signatures have been verified by a handwriting expert.

Signature expert Richard Westwood-Brookes called the item a “one off”, adding: “I’ve been in this business a long time and I’ve dealt with Charles and Camilla material many times.

“I verified the signature on the box against examples that are available. I’ve had Camilla letters in the past and it was easy to identify her signature.

“This is an interesting piece of memorabilia. It shows what a tragedy it was that Charles didn’t marry Camilla all those years ago. This shows you the human face of the Royals. We’ve all had family parties where we played games and scribbled on things. This is a real insight into them as people.”