Princess Diana's naughty humour revealed in old birthday card

A naughty card sent from Princess Diana has been put up for auction by LA-based house Julien's Auctions, after the anonymous owner decided to sell it.

The late royal, who died in 1997, reportedly sent the X-rated birthday card to her friend and personal accountant, Anthony Burrage.

The outside of the card shows Sleeping Beauty, with the caption: 'A little prick in the hand sent Sleeping Beauty to sleep.'

Inside, it reads: 'Only the big ones are worth staying awake for!'

Princess Diana signed the card: 'Tony, a belated Happy Birthday for the 5th. From, Diana.'

According to the auction house, the card is worth up to $AUD1,627.

'I know that Diana liked to send humorous cards, but I haven't seen any quite this naughty,' the owner told the Mirror.

'So for me it really shows that she was a fun human being that could share some dirty jokes with her closest friends.'