The Queen loves to spot mistakes in Downton Abbey

Queen Elizabeth II. Photo / Getty Images

When Downton Abbey returns to screens for the last time later this month, one hawk-eyed fan in particular will be ready to pounce on any historical inaccuracies the producers allow to slip through the net.

The Queen, it has emerged, is a huge fan of the series, and loves nothing better than to point out the mistakes that have become a whole sub-plot in themselves for devotees of the show.

Brian Hoey, the author of At Home With The Queen, revealed that Downton came up in a recent conversation with a worker at Buckingham Palace.

The cast of Downton Abbey. Photo / Supplied

He told The Daily Telegraph: "We were talking about Trooping the Colour and how the Queen always notices if anything is not quite right, such as one young officer she spotted wearing his medals in the wrong order not long ago.

"Apparently she is exactly the same when she is watching the television. She loves watching Downton Abbey and pointing out things they have got wrong, partly because she is familiar with Highclere Castle, where it is filmed. She used to stay there as a guest of the Carnarvon family.

"She is the same when she is watching anything on television. In one programme she was watching, the Queen noticed that a British officer was wearing medals that were from the wrong era. It was set in the First World War but the medals he was wearing did not come in until the Second World War."

The Queen is not the only royal Downton watcher - the Duchess of Cambridge is known to be a "huge" fan, and makes the Duke of Cambridge watch it with her, and the Duchess of Cornwall never misses an episode, though the Prince of Wales is less keen.

Downton Abbey employs the royal commentator Alastair Bruce as its historical adviser, but his advice is not always heeded and a string of historical anomalies have cropped up.

In an episode set in 1912, a 1921 Model T Ford was being used. In another, there was a piano rendition of a song written six years after the episode was set. Such late 20th century devices as television aerials, double yellow lines and a PVC conservatory have also made appearances, and in a publicity photograph for the fifth series a plastic water bottle had been left on the fireplace.

Mr Hoey said the Queen also loves watching Dad's Army, perhaps because she remembers the Home Guard platoon based at Windsor Castle during the war. She is also a fan of Last of the Summer Wine and, surprisingly, the X Factor.

During her visit to Ireland in 2011 she told Mary Byrne, a former contestant on the ITV1 talent show, that her staff record X Factor for her and she watches it the next day.