Putonghua-speaking fans of BBC TV's super-sleuth Sherlock Holmes to ponder special clue in New Year's Day episode

Putonghua-speaking fans of the BBC TV series will don their deerstalkers when the latest episode airs in the UK and America on January 1 to decipher a special clue that only they will be able to unravel.

Writers and producers of the series about super sleuth Sherlock Holmes - played by Benedict Cumberbatch, with Martin Freeman as his sidekick, Dr Watson - have included a special Putonghua reference in a special, one-off episode called , which takes the show back to its Victorian-era roots.

"It's a new story, but if you know the original stories, you'll see it's fashioned out of quite a few others," said the show's writer Steven Moffat, referring to Sir Arthur Conan Doyle's novels. "We've chosen several [stories] and there are loads of references. One of them you have to be able to speak Chinese to get."

While the show has a legion of loyal fans worldwide, those in China have adopted the duo with such gusto that the pair have been given nicknames based on how their characters' names sound in Putonghua. Holmes is known as Curly Fu, while Watson is called Peanut. Both terms are hugely popular on mainland social-media sites such as Weibo.

Fan clubs for the show have sprung up in major cities across the mainland, and it was the first country to access episodes of the recent series, just hours after it aired in Britain, media reported.

A BBC spokesman told the that while the special Chinese reference was not a direct message to fans in China, its popularity in the region was recognised.