He is the acting heavyweight known best to a generation as Harry Potter's headmaster, Albus Dumbledore, while she is the Welsh classical mezzo soprano who has barely any TV credits to her name.

Now, Michael Gambon and Katherine Jenkins will add a bit of time-travel to their CVs when they guest-star in this year's Doctor Who Christmas special.

Billed by screenwriter Steven Moffat as "the most Christmassy Christmas special," details are nonetheless being kept under wraps by the BBC, which has confirmed only that the show will be a "clever twist on Charles Dickens' Christmas Carol."

Since its revival in 2005, Doctor Who has proved one of the biggest family hit shows in the UK, and the Christmas special has become one of the highest-rating Christmas Day shows. A previous Christmas special featured Kylie Minogue.