Unseen image shows Kate Middleton taking picture of boyfriend Prince William at 2006 Sandhurst polo match - four months before she made first official outing alongside the Royal Family at passing out parade

An unseen image of Kate Middleton with Prince William at a Sandhurst polo match four months before she made her first official appearance with the Royal Family has emerged. 

The picture was taken by American couturier and bridalwear designer Pat Kerr Tigrett - who became a friend of the late Princess Diana - on July 13, 2006, after she received an invite to watch the charity fixture. 

It shows the then 24-year-old William on horseback in polo gear at the Royal Military Academy Sandhurst, in Berkshire, as his girlfriend of three years takes his picture.

In December that year, Kate - now the Princess of Wales - joined Queen Elizabeth II, Prince Philip and the then Prince Charles at William's passing out parade at Sandhurst.

She beamed alongside her parents Carole and Michael and wore a scarlet coat and black hat which matched the colours of William's uniform. 

The official appearance was billed as the strongest sign yet that the couple would marry - but they then split up in April 2007.

Royal fans were bereft but quickly received a boost when the couple got back together a few months later and then married in 2011. 

Ms Tigrett is seen in another newly-revealed image posing alongside Prince William - who she says gave her his trophy - during the day out at Sandhurst. 

An unseen image of Kate Middleton with Prince William at a Sandhurst polo match four months before she made an official appearance with the Royal Family has been revealed by a designer who befriended his late mother Princess Diana. Above: The image from July 2006 showing Kate all in white taking a picture of William on horseback. The photo was taken by American couturier and bridalwear designer Pat Kerr Tigrett 

Ms Tigrett is seen in another newly-revealed image posing alongside Prince William - who she says gave her his trophy - during the day out at Sandhurst 

The designer revealed the images in an interview with MailOnline to mark the sale of three of the late Princess Diana's dresses.

Ms Tigrett is selling them nearly 30 years after she bought the garments at the famous Christie's auction in June 1997, where Diana sold off 79 of her evening gowns to raise money for charity. 

The designer said of the Sandhurst visit: 'This lovely invitation arrived from Prince William to attend a polo match at Sandhurst.

'This beautiful girl was taking photographs of him, and it appears that Catherine and the Prince knew each other.

'It was a wonderful occasion and, of course, Prince William won. He then entered the VIP marquee holding his trophy.

'We had such a nice conversation about his mother and her ancestral home Althorp. To my surprise he handed me his trophy which I truly cherish.'

Ms Tigrett first met Diana in 1984 when she lived in London's Regent's Park with her husband John Tigrett, a Tennessee entrepreneur. 

In December 2006, Kate joined Queen Elizabeth II, the Prince Philip and the then Prince Charles at William's passing out parade at Sandhurst. It was her first official appearance alongside the Royal Family. Above: Kate seated next to her mother Carole and father Michael and William's friends 

Kate walks in between her mother and father at William's passing-out parade at Sandringham in December 2006

Queen Elizabeth II enjoys a funny moment with her grandson Prince William as she observes him during his passing out parade at Sandhurst

She was introduced to the Princess at the home of financier Sir James Goldsmith and his third wife Lady Annabel. 

'When I first met Princess Diana, she was with Prince William. It was before Harry was born,' she said.

'I thought: "How young she is, how young to be taking on such an incredibly impossible responsibility as the future Queen of England".

'But she was totally charming and was such fun and she adored her boys.'

Over the next 13 years, Ms Tigrett and Diana saw each other again at balls and functions.  

Ms Tigrett paid a total of £80,241 for four of Diana's dresses: a Murray Arbeid midnight blue ballerina-length tulle gown; a Victor Edelstein magenta silk and lace dress; a cream silk Catherine Walker dress and a black velvet Catherine Walker gown with a lace collar. 

The car pass for the July 2006 charity polo match at Sandhurst Royal Military Academy 

American couturier and bridalwear designer Pat Kerr Tigrett (pictured above in 2017). She bought four of Diana's dresses at the famous 1997 Christie's auction. The pair had known each other since 1984

She is now selling all but the Catherine Walker black velvet dress. 

The garments are being sold via Julien's auction house in Los Angeles on June 27. 

Diana was killed in a car crash in Paris at the end of August in 1997.

Ms Tigrett said: 'The phones were ringing like crazy, and I just cried. It was so awful. She was so well loved by so many and those children of hers were her life. 

'The minute she died, I literally put my gowns away.' 

The following year, Ms Tigrett was invited by Diana's sister Lady Sarah McCorquodale to make a documentary about Diana's wardrobe.

The film was to accompany the exhibition Diana: A Celebration, which was held at the Princess's ancestral home Althorp Park, in aid of the Diana, Princess of Wales Memorial Fund.

Ms Tigrett said she was taken to Althorp, where she was shown Diana's wedding gown and her grave, which lies on a small island on the estate.

'I felt so blessed to be able to say goodbye,' she said. 'I took photographs of where she was buried, thought about her, prayed for her and read all the wonderful messages written by her family.'