The Queen’s reputation as a timeless, dignified dresser immune to fashion has been transformed by a photograph of her in full embrace of 1970s chic.

A previously unpublished picture shows her dressed in a pair of blue flared trousers with a well-defined crease down the front and a vibrant floral jacket that may owe its inspiration to a Persian rug. She is grinning broadly and clutching a pair of sunglasses while standing on an unknown beach. The Royal Yacht Britannia, moored in the background, suggests that she is abroad, but the location is unknown.

The photograph has been made public before it is offered for sale by Reeman Dansie auctioneers in Colchester, Essex. It is being sold by a relative of the late Marchioness of Cambridge, who accompanied Her Majesty on several overseas trips.

Images of the Queen in trousers are almost unknown. Mark Stewart, a royal photographer for 20 years, said that he had only once seen her in slacks — when she emerged from hospital in a trouser suit after a knee operation in December 2003.

The Times photographic archive features only three other images of the Queen in trousers. Two were taken during seagoing voyages, when windy conditions perhaps brought to mind Marilyn Monroe’s billowing skirts in The Seven Year Itch. The first is a photograph aboard Britannia in the 1950s or 1960s. The second is from 2006, when the Queen visited the Western Isles aboard MV Hebridean Princess. The other photograph, from 1974, shows the Queen in jodhpurs.

Not one of these pairs of trousers resembles flares, which are especially unsuitable for equestrian pursuits.

James Grinter, who will auction the photograph on Monday, said that the print was strikingly different from the public perception of the Queen. “I should imagine it was in the 1970s, judging by her attire,” he said. “It is nice to see that she has got her little paddle boat out at sea — the Royal Yacht Britannia in the background.”

The photograph will be sold in the same lot as one of the Duke of Edinburgh, thought to have been taken in the 1940s, that shows him larking with his family in Milford Haven. The Duke is shown in a chain of Mountbatten family members sitting on one another’s knees. “They look like they’re enjoying themselves. I should imagine it was in the 1940s,” Mr Grinter said.

The lot is estimated to fetch up to £60, although Mr Grinter said that this was a “come and buy me” estimate. “I hope that they will sell for more than that, but it is very difficult to put a price on these things.”

The sale will also include images from a collection acquired by the late Sidney Cumper, who served Queen Mary from 1925 until her death in 1953. They show the Queen as a little girl, sitting on a slide with Princess Margaret and posing next to a classical monument. Another shows her poised to throw a snowball at her nanny, Margaret MacDonald, affectionately named Bobo. The three photographs are expected to fetch up to £600.

Ms MacDonald fell from favour when she published details of the Queen’s upbringing in her memoirs, Mr Grinter said. “It was very frowned upon. She was blacklisted.”

He said that royal memorabilia had become a growing market since an auction of possessions owned by William Tallon, Queen Elizabeth the Queen Mother’s servant, who was affectionately known as Backstairs Billy.

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