Oscar-winning screenwriter Fellowes has based the storyline for the new movie - a spin-off from the popular ITV period drama, which ended in 2015 - on a royal visit by King George V, Queen Mary and their entourage to Downton Abbey, where they are to be guests of the Crawley family.
He has now admitted that he was inspired to write the script by a visit the royal couple made in 1912 to Wentworth Woodhouse, near Rotherham, as guests of the Earl and Countess Fitzwilliam.
Wentworth Woodhouse restoration reveals secrets of an unexplored rooftop worldFellowes told Vanity Fair that he had been reading about the royal tour to Yorkshire, which came during a time of increasing industrial tensions in Britain. The King wanted to appear visible to his subjects, and decided to spend four days visiting collieries and factories in one of the country's manufacturing heartlands.
Fittingly, the Downton Abbey producers paid tribute to Wentworth's role in the new film - which hits cinemas on September 13 - by using the great house as a filming location.
During the King's stay, a ball was held in the Marble Saloon, and the ballerina Anna Pavlova performed for the royal party. This dance was recreated by the crew, who recruited over 150 extras and a band to appear in the scene, which can be seen in the movie trailer. The Marble Saloon has suffered from dry rot and water damage during the years since the Fitzwilliams sold their ancestral seat, but retains much of its magnificent grandeur.
The film is set in 1927, when King George V was still on the throne - the grandson of Queen Victoria and grandfather of our present Queen Elizabeth II reigned from 1910 until his death in 1936.
Unpaid volunteer who saved Wentworth Woodhouse for the nation is given a damehoodFellowes believes that a royal tour and country house visit would still have been appropriate in the inter-war years, when many of the European royal families were reeling from social unrest and revolution which threatened the future of monarchical rule.
As the fictional Downton estate is in Yorkshire, he also felt the 'parallel' between the two events was appropriate. Royal parties travelled with a large entourage of servants, private secretaries, equerries and other retainers, and Fellowes saw the visit as providing plenty of material for a multi-faceted plot involving rivalries between the King's and the Crawleys' staff, who would have been challenged to host an occasion on an almost overwhelmingly lavish scale.
What happened during the 1912 royal visit to Wentworth Woodhouse?
King George V had apparently been considering spending the summer of 1912 visiting his cousins in the various royal houses of Europe - but was persuaded to undertake a tour of the Yorkshire coalfields instead, in an effort to connect with the industrial working class.
Realistically, there weren't many stately homes that were large and grand enough to host the King and Queen that were also within reasonable travelling distance of collieries, foundries and factories - Wentworth was the obvious candidate. Nearly 80 of the bedrooms were required to accommodate the party.
Among the local aristocracy and gentry invited to join the Fitzwilliams for the visit were the Earl and Countess of Harewood, whose son and heir would later marry the King's only daughter, Princess Mary.
Meet the volunteers restoring Wentworth Woodhouse's historic gardens to gloryThe couple visited coal mines, and huge crowds gathered on the streets to wave at the royal procession as it passed through. The King even descended underground at one pit.
The tour culminated in a musical performance on the lawns in front of Wentworth, when 25,000 people crowded into the park hoping for the glimpse of the royal couple on a balcony. The National Anthem was sung and the King gave a speech.
Filming Downton Abbey at Wentworth Woodhouse
"It was like stepping back in time".
Tracy, a volunteer at Wentworth Woodhouse, speaks in awed tones when she recalls the day Downton Abbey came to Wentworth last year.
She looked on as some of the most famous faces in British television drama crowded into the Marble Saloon, perhaps the most opulent room in the whole mansion.
They stayed in Tankersley Manor Hotel while shooting one of the movie's most stunning scenes - a ball.
Over 150 extras dressed as footmen and waiters crammed into the room and a band played for the cast as they danced.
These Yorkshire locations appear in the new Downton Abbey filmBack in 1912, the Marble Saloon, with its sprung dancefloor and galleried landings, was the focal point of the programme of entertainment laid on for King George V and Queen Mary. The most famous ballerina of the age, Anna Pavlova, was invited to perform.
The other Yorkshire filming locations
Another scene that appear in the trailer shows a couple dancing on an outdoor terrace at Harewood House, near Leeds.
Harewood was a house of similar stature to Wentworth in 1912, as both were the seats of an Earl.
Harewood also had close links to the Royal Family in the 1920s, when the film is set.
By this time, Princess Mary, the King's daughter, was married to Henry Lascelles, heir to the Harewood earldom - he would inherit the title in 1929. They had two sons.
The marriage meant the King and Queen were regular visitors to both Harewood and the couple's first marital home, Goldsborough Hall, near Knaresborough.
The Lascelles family were also in the Fitzwilliams' social circle.
Further filming took place at Pickering Station on the North Yorkshire Moors Railway and in the historic Little Germany area of Bradford.
The Fitzwilliam family rented Wentworth out after World War Two, and it has also had two private owners.
After the death of the most recent owner, the house was purchased by the Wentworth Woodhouse Preservation Trust in 2017. The charity aims to fully restore the house and gardens and open them to the public for the first time.
Currently, pre-booked tours of limited areas of the house and grounds are available, and there is a cafe and gift shop.