This promotional picture for the new series of Downton Abbey caused an online sensation when a sharp-eyed fan noticed the very modern bottle of water on the mantelpiece behind Hugh Bonneville and Laura Carmichael.

The new series – the fifth – launched on Thursday and Bonneville and Carmichael, who play the Earl of Grantham and Lady Edith respectively, found themselves upstaged by the bottle in the background.

One fan, Diana Pearl, offered a possible solution, tweeting: “Obviously the Crawley family invented the water bottle, and that’s how they were able to support the estate.”

It is not the first historical blooper, with previous episodes featuring television aerials, double-yellow lines and even a modern conservatory.

The new series of Downton is set in 1924 and opens with the election of the first Labour government – news which divides the house and its inhabitants.

The ITV costume drama, which has been a huge critical and commercial hit in this country and beyond, employs its own expert, Alastair Bruce, who is in charge of historical accuracy and etiquette.

Downton Abbey 5 will launch in the autumn.