Over the past three years, viewers of Downton Abbey have become familiar with the lavish entertainment and formal etiquette of glamorous high society functions.

But this year’s Christmas special will venture into even more rarefied circles, The Mail on Sunday can reveal, with a storyline that features royalty for the first time.

The plot, which producers had hoped to keep a closely guarded secret, will revolve around Lady Rose’s debutante ball, when she will be presented to King George V and his wife Queen Mary at Buckingham Palace.

But despite being unveiled as one of society’s most eligible young ladies before a host of wealthy bachelors, the moment is likely to prove bittersweet for Rose, who is already in love with jazz singer Jack Ross, the show’s first black character.

The show is currently in its fourth season, airing in the UAE on OSN, but insiders on the show are now claiming the fifth season may be the last, with some of the cast members said to be “keen to go out on a high”.

The UK’s Sunday Express reports that TV channel ITV “are talking about recommissioning, definitely for a fifth, but some of the senior cast members want to call it a day at five.”

Elaborate sets

Millions are expected to tune in to ITV’s Christmas Day episode, which show insiders claim is the “most spectacular, ambitious and glamorous” yet, due to the elaborate sets which needed to look “fit for royalty”. The scenes were masterminded by show creator Lord Fellowes alongside historical adviser Alastair Bruce — who also makes a cameo appearance as the Lord Chamberlain.

Fellowes said the storyline was inspired by his own memories of the balls. “We thought it would be a fun story to tell,” he said.

“We all know that Mary, Edith and Sybil have been through this process, but we felt that many of the audience won’t know what the process is, and why should they? Girls were presented twice — first as a debutante, then as a bride. In the evening, you got dressed up in specific costumes.

“Tiaras would not be worn by unmarried girls, but they would wear three ostrich feathers and a veil which descended on to a train.

“The girls would sit in The Mall in a line of cars and people would gather and practically give them marks out of 10. I can remember friends of my mother’s would use our house to get changed, and the girls would come down the stairs in their floating, bouncy skirts, and ravishing picture hats. I was only about six but I still remember how lovely they looked.”

Shock plot twist

Last year’s Christmas special caused a sensation with the death of Matthew Crawley, with some fans even complaining that the shock plot twist “ruined” their Christmas Day.

But some may be disappointed that this year’s festive special will again not be set at Christmas, instead taking place in spring.

Lady Rose, played by Lily James, will be escorted to the ball by her aunt, Lady Grantham.

A Downton source said: “This is the most spectacular episode we have ever produced.”