The Duchess of Cambridge visits the set of Downton Abbey at Ealing Studios

The world-wide success of Downton Abbey is all thanks to The Duchess of Cambridge - the show's creator has revealed in west London. Lord Fellowes told The Duchess 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 Her Royal Highness 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.

The Duchess of Cambridge, 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. The Duchess 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 The 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: "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."

The Duchess was taken on a tour of the various departments and met cast and crew, on her visit to Ealing Studios to celebrate the success of Downton Abbey; one of Britain’s greatest cultural exports.

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, The Duchess of Cambridge 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, The Duchess 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, The Duchess looked amazed as executive producer Chris Croucher told her they aim to film just four minutes of screen time a day.