Queen Camillas grandmother has been revealed as an unlikely pioneer of dressing up for Halloween - alongside Princess Dianas step-grandmother Barbara Cartland.
Sonia Cubitt, née Keppel, is seen in newly-unearthed photographs modelling a spooky Three Candles costume for a Halloween ball back in October 1930.
She was one of 13 society It Girls of the era who donned outfits specially designed by novelist Dame Barbara for what was dubbed a Pageant of the Superstitions special at Londons Park Lane Hotel.
The women - including Dame Barbara herself - appeared at midnight on October 31, 1930 at the ball, which has since been described as Britains first major fashion showcase of its kind.
It was an early example of people this side of the Atlantic putting on Halloween party attire.
The photos have emerged as modern-day revellers across the country - as well as celebrities at elaborate parties - will be doing similar tonight for Halloween.
The list of models and their outfits titles were shared by Dame Barbaras granddaughter Tara McCorquodale, who told MailOnline: My grandmother loved to dress up and designed many wonderful creations including all the sensational dresses at the Superstitions Ball.
Always an optimist, she took no chances and went as "Good luck" - contributing to the invention of Halloween costumes, as we know them today, is an accolade she would have worn proudly.
Queen Camillas grandmother Sonia Cubitt, née Keppel, modelled a Three Candles-themed costume for a Halloween ball in London in October 1930, newly-unearthed photos show
The outfits for the Pageant of the Superstitions were designed by romantic novelist Barbara Cartland, seen here wearing the collections Good Luck outfit
Sonia Cubitt was the grandmother of Queen Camilla, seen here last Saturday during the visit she and her husband King Charles made to Samoa
Sonia was one of the daughters of Camillas great-grandmother Alice Keppel, who had been the mistress of King Edward VII.
Dame Barbara was the mother of Raine Spencer, who became Princess Dianas stepmother when she married her father Earl Spencer in 1976.
The author died aged 98 in 2000. More than 750million copies of her books have been sold around the world.
In 1930, she was tasked with coming up with 13 outfits that would give a quirky spin on unlucky concepts.
The clothes were made by the theatrical costume company L & H Nathan Ltd.
Sonias was inspired by the superstition that three candles should never be lit as the person closest to the shortest one would supposedly then be the first to either marry or die.
Dame Barbara herself - then described in contemporary reports as Mrs A. G. McCorquodale, in reference to her then-husband - wore an over-sized horseshoe on her head to symbolise good luck.
Other models included Miss Mary Pitcairn as a witch, the Countess of Carlisle as a broken mirror, the Hon. Mrs Parsons marking the number 13, Lady (Hamilton) Grant as Upsetting The Salt and the Hon. Mrs John Russell depicting Friday.
The Princess George Imeritinskys defining feature was being green, Miss Carl Bendix represented a single magpie while Miss Betty Hultons outfit was based on crossed knives.
A preview showing of the outfits was held on October 10 1930 at Londons Haymarket Theatre
Among the It Girls of the era taking part was Princess George Imeritinsky, or Avril Joy Mullens, wearing a costume symbolising Green
Princess George Imeritinskys costume was dominated by elaborately large horns
Dame Barbara Cartland, pictured in 1995 on ITVs The South Bank Show, was a bestselling romantic novelist as well as step-grandmother to Diana, Princess of Wales
Meanwhile, Mrs E. De Winton Wills was wreathed in peacock feathers, Mrs Edward Baron was said to symbolise The Moon Through Glass and the Hon. Mrs Evan Morgan was given a dress dubbed Opals.
Before the pageant itself, the It Girls posed in the costumes for renowned photographer Alexander Sasha Stewart at a special charity matinee performance.
This event, at the Queen Charlotte Maternity Hospital Maintenance Fund, was held at central Londons Haymarket Theatre on October 10 1930.
The Halloween pageant itself attracted plenty of media coverage at the time, including a report in the following days London Daily News on page seven.
The newspaper wrote: At the Halloween ball at the Park Lane Hotel there was a midnight pageant of the superstitions, in which society woman in gorgeous costumes represented "Upsetting the Salt", "Crossed Knives", "Friday", "The Moon Thru Glass", "Broken Mirror" and many other omens of bad luck.
The Western Daily Press had previously reported ahead of the occasion: Arrangements are now complete for the "Pageant of the Superstitions" which is to be the feature of the Halloween Ball at the Park Lane Hotel, London on Friday, October 31. in aid of the Towns Womens Guilds Appeal.
Well-known society women will, for one evening at least, defy superstition and represent 13 - ominous number - of the chief superstitions.
Mrs Carl Bendix donned a Single Magpie costume for the Pageant of the Superstitions
Aso taking part was Sylvia de Winton Wills wearing a Peacock design
Another socialite who joined the Halloween dress-up was the Hon. Mrs John Ballan, given a Broken Mirror costume for the Pageant of the Superstitions in October 1930
At the time Sonia was widely known as Mrs Ronald Cubitt, having married the former Coldstream Guard nine years earlier.
He was the heir to Ashcombe Barony in Dorking, Surrey, and a fortune built by his Victoria great-grandfather Thomas Cubitt.
As well as being a renowned socialite about town, Sonia was an author - publishing books including Three Brothers At Havana, 1762 and The Sovereign Lady: A Life of Elizabeth Vassall, Third Lady Holland, With Her Family, as well as her 1958 memoir Edwardian Daughter.
The couple had three children including eldest daughter Rosalind who herself would also have three children, the first being Camilla who was born in 1947.
Sonia, who was divorced from Roland in 1947, died aged 86 in 1986 after suffering from chronic osteoporosis.
Camilla became a member of the National Osteoporosis Society eight years later - the same year her mother died after also suffering from the skeletal disorder - and became the charitys patron in 1997, then its president in 2001.
The Halloween 1930 photographs have been tracked down in the Getty Archives by a team at Faircado, an AI-powered tool to help people find second-hand alternatives when searching for fashion and other products online.
They are encouraging people to recycle costumes when dressing up for Halloween this week, with estimates suggesting 7million October 31 outfits are thrown away in the UK each year - potentially damaging the environment.
The superstition theme given to Lady Hamilton Grant, or Margaret Cochran, had the title Upsetting The Salt
A Crossed Knives look was the one for Betty Hulton, with clothes made by L & H Nathan Ltd
The showcase attracted plenty of Press coverage, including here in the Daily Sketch
Evolena de Wilde dEstmael, co-founder and CEO of Faircado, said: Dressing up for Halloween is great fun, but whats truly scary is the impact it can have on the planet.
Many people buy fast-fashion costumes, wear them once, and then throw them away to languish in landfill.
These items are cheaply produced and often made from plastics, leaving a footprint of CO₂ emissions.
Barbara Cartlands witty designs prove that a lot of the joy that comes from a good costume is showcasing your imaginative thinking, and that moment when other party-goers "get" your costume.