The world-wide success of Downton Abbey is all thanks to the Duchess of Cambridge - the show's creator has revealed.
Lord Fellowes told Kate she was the reason the ITV period drama had taken off overseas, after she unwittingly plugged the show during a visit to the US in 2011.
He praised the Duchess as a "functioning member of the Downton team" after she had been taken on an extensive tour of the show's sets at the world famous Ealing Studios in West London.
Kate, who is a fan of the series, clearly enjoyed her behind the scenes visit meeting cast and crew who included Hugh Bonneville who plays Robert, Earl of Grantham and Jim Carter, the one of the leading figure below stairs, butler Mr Carson.
She revelled in the period detail of the costumes, sets and props used in the drama now set during the reign of George V - asking dozens of questions as she visited make-up and hair departments, toured scenery construction areas and watched live filming.
After joining the cast for the group photo in the servants' hall she told the actors: "Thank you so much for letting me into your world."
At the end of the visit Lord Fellowes told the cast, crew and Duchess: " I hope you've seen the pleasure you've given by coming here this morning, we really do appreciate it. It's been rather an extraordinary journey aboard the good ship Downton but this is certainly one of the high points of the voyage."
He highlighted how the Crown had been involved in the theatre over the centuries since King Charles II ordered actresses be allowed to appear on the London stage.
Lord Fellowes added: "There is an extra detail that perhaps some of you might not be aware of, your Royal Highness has played a significant part in the Downton story already.
"You may know that one of the hardest things about selling a new show or movie or anything is just letting the people know it's there.
"It's just making the public aware of the fact that this show has been made, they should watch it, they might enjoy it. And it's a tough task. It's very important.
"And it so happens that just as we were trying to shift Downton in America, you actually made a visit and someone said, had you ever heard of a show called Downton Abbey. And you said you'd heard of it, you'd watched it, you'd enjoyed it.
"This went into the media like an Exocet missile and soon all of America and most of the world knew it and as a result our challenge faded away."
Wearing a cream coat by British maternity label JoJo Maman Bebe, Kate was taken on a tour of the various departments and met cast and crew.
Her first stop was the hair and make-up trailer, where she chatted to Phyllis Logan, who plays housekeeper Mrs Hughes, as she was prepared for the cameras by make-up artist Nic Collins, 44.
Also in the small space was Rob James-Collier, who plays under-butler Thomas. He pointed at the actress's forehead and joked: "You've come in at a really good point because Phyllis is about to have her botox put in - it's a key point in her morning arrangements."
Moving into the props room, Kate was shown a range of items that feature in the programme, including handwritten letters, newspapers, china and cutlery.
She told prop master Tom Pleydell Pearce: "It's so fantastic, you've got so many objects. There's a lot of attention to detail."
Turning to his team, she said: "Well done, everyone."
During the tour the Duchess watched a scene being filmed for the latest series - Downton's sixth.
In the dimly lit bedroom of Lady Mary Crawley, played by Michelle Dockery, Kate watched the aristocrat and her maid, Anna Bates, played by Joanne Froggatt, film a scene.
Standing still and surrounded by crew, she watched as Anna helped her mistress dress for a night out.
After they had finished, Kate chatted to the actresses and asked them: "Do you get the giggles?"
They replied in unison: "Oh yes."
The Duchess then joined senior production staff to watch a second take of the scene on monitors outside the bedroom.
Wearing a pair of headphones customised with the initials HRH, Kate looked amazed as executive producer Chris Croucher told her they aim to film just four minutes of screen time a day.
In the servants' hall, she met Brendan Coyle, who plays Bates, alongside the panel of bells used to call staff on the show.
Many interior scenes are filmed in the studios because kitchens and staff quarters at Highclere Castle, where the series is also shot, have been modernised.
Kate said: "You can't tell when you are watching that anything is filmed in a studio."
In Mr Carson's study, actor Jim Carter showed her a cupboard of silverware and joked: "This is it, where we lock up the silver. I don't let anybody in here. Many of the cast were saying they have never been in here."
He added that his favourite scenes were filmed there, "particularly when the lights are down and I'm having a glass of sherry with Mrs Hughes".
Kate replied: "The sets are so much smaller than you imagine - especially when you've got all the crew in here as well."
Pointing to a stuffed fish in a glass case known as "Billy the Bass", Lord Fellowes told her: "This is Carson's one moment of pleasure-seeking - it's the only fish he ever caught."
Kate laughed and said: "I'll have to keep an eye out for that."
In the impressive period kitchen set the Duchess found Mrs Patmore, played by Lesley Nicol and Daisy (Sophie McShera) who presented her with a Downton chocolate cake and divulged some show secrets.
The cast members joked about how a fridge in the corner of the room was just a prop. Ms McShera said: "It's got a really bad smell - it's not real."
They also showed Kate how some of the cupboards would not open and joked about food props being used again and again, including a lobster.
Kate, whose second child is due next month, received a number of gifts during her visit including a wooden train for her son Prince George and a bound copy of the script from the first episode of series one, signed by the cast.
After Bonneville had met Kate at the reception he joked about the royal visit, saying: "It meant we've had to pull our socks up but luckily for the past five years we've been practising our Ps and Qs.
"But it's been wonderful to have the visit to Ealing studios, the oldest studios still running in the world."
Asked to explain Downton's enduring appeal he replied: "It's compelling characters, humdinger of a plot and if you get bored of a character, there is another 28 seconds later."
Kate, who was visiting the set in recognition of the show's place in British TV success around the world, may have been a little excited to be walking on the Downton set, but some of the cast were more in awe of her.
Laura Carmichael, who plays Lady Edith Crawley, said: "Brendan (Coyle) and Hugh were fighting over who was her favourite character. But she didn't reveal."