Today, they are kept under wraps, far from photographers’ flashbulbs, in temperature-controlled rooms where they form part of a royal clothing archive dating back 500 years.

Yet three of Princess Diana’s iconic dresses were only saved for the nation after they were rescued from a second-hand shop in Hampshire.

Stranger still, they had been taken there by no less a figure than the mother of Sarah Ferguson, Duchess of York.

Quite how or why remains a mystery. But as I reveal in my book Diana: A Life in Dresses, the late Princess was surprisingly relaxed about her clothes, despite her love of fashion.

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Wearing a tartan day dress to the Braemar Highland Games in September 1982. Precious items were routinely discarded or abandoned. Diana was generous, too, giving many away to friends and family

There was also a tartan coat (pictured) created for the Princess’s 1985 visit to Italy by the Emanuels, the British husband-and-wife team who made Diana’s wedding dress

Precious items were routinely discarded or abandoned. She was generous, too, giving many away to friends and family. The fact that so many survived for posterity has often been down to outrageous chance.

The three outfits in question now sit in the Royal Ceremonial Dress Collection storage facility at Hampton Court after that mysterious intervention by Susan Barrantes.

A source confirmed to me that Barrantes had handed three of them over to an upmarket resale shop in Hampshire at some point in the 1990s.

These were a green tartan day dress that the Princess wore to the Braemar Highland Games in September 1982, and a turquoise Catherine Walker dress she took on a royal tour of Australia and New Zealand in April 1983.

There was also a tartan coat created for the Princess’s 1985 visit to Italy by the Emanuels, the British husband-and-wife team who made Diana’s wedding dress.

From the Hampshire clothes shop, they went to auction before being purchased by Historic Palaces England.

Did Diana give them to her friend Sarah Ferguson as hand-me-downs? Had they simply been left lying around at the Duchess of York’s Sunninghill Park estate in Berkshire?

Was Ferguson’s mother for some reason asked to sell the outfits on Diana’s behalf – and was it just the three dresses?

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Princess Diana in a blue Catherine Walker outfit on tour of Australia and New Zealand in 1983

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Was Sarah Ferguson’s mother for some reason asked to sell the outfits on Diana’s behalf – and was it just the three dresses?

Sadly, Susan Barrantes died in 1998, less than a year after the Paris car crash that killed Diana and the vintage shop in question has yet to be identified.

It’s not the only time that a provincial second-hand shop has come to the rescue of fashion fans.

An Emanuels-designed ivory silk moss crêpe evening gown, which Diana had worn in Bahrain in 1986 during a lavish banquet in her honour, ended up in a clothes shop in Hereford.

Diana is believed to have given the dress to her childhood friend, Caroline Twiston-Davies.

In 1996, her housekeeper took the dress – along with ‘more mundane daywear and suits’ – into Chameleon, a vintage clothes shop near Caroline’s family estate.

As I explain in my book, it was initially bought a few months later for £200 by the shop’s part-time sales assistant, who had intended to wear it herself to a hunt ball. Instead, it remained unworn, packed away in a box, until she realised its historical significance.

It was sold at auction in 2018 for £162,500.

For the announcement of her engagement to Prince Charles in February 1981, ‘Shy Di’ had wanted an outfit to match her sapphire sparkler – and had hoped to find something in Bellville Sassoon, a high-end Knightsbridge salon.

However, the imperious French shop assistant failed to recognise the nervous teenager as the future Princess of Wales, and was cross she’d come in so near to closing time.

‘It was 5pm at night and this little girl came in and didn’t know what she wanted,’ couturier David Sassoon explains. ‘The assistant said: “Do you want a dress? A suit?” Diana said: “I’m not sure.”

‘She showed her a few things, and Diana didn’t think they were right. So, she suggested Diana went to Harrods.’

Which she did: there, she bought a cobalt blue skirt suit by British label Cojana that she wore to face the world’s press on the steps of Buckingham Palace.

The black taffeta strapless ball gown that Diana wore for her first public engagement with Prince Charles in March 1981 – and which she famously ‘nearly fell out of’ as she stepped out of their car – was kept for decades in a bin bag and forgotten about.

It had been returned to the Emanuels for taking-in as Diana has lost a great deal of weight in her early married days.

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The Duchess Of York With Her Mother, Susan Barrantes, who gave some of Princess Diana’s dresses to a second-hand shop

However, instead of altering it, the Emanuels decided to make a new version from scratch, and placed the original in a bin bag for safe keeping.

It was only found years later, in a corner of the designers’ Mayfair atelier.

In 1997, just two months before her death, Christie’s had held a New York auction of 79 dresses, raising £1.96 million for her favourite AIDS charities.

Many of these are now in public hands.

Today, the long blue-and-white dress 18-year-old Diana wore to her coming-out ball at Althorp, the Spencer family estate, was bought by a neo-natal nurse from Utah. It is now owned by a fashion museum in Chile.