Doctor Who fans have wondered for years if the sci-fi drama would have to come to an end due to the 12 regeneration rule.
However executive producer Stephen Moffat, who announced that Matt Smith is the '13th Doctor', has suggested the riddle will be solved on Christmas Day, reports the Daily Mirror.
He told the newspaper: 'The 12 regenerations limit is a central part of Doctor Who mythology. So if the Doctor can never change again, what's Peter Capaldi doing in the Christmas special?'
The Day of the Doctor: Matt Smith, John Hurt and David Tennant in the 50th anniversary special
The writers have left viewers guessing after all 12 Doctors lined up during Saturday night's 50th anniversary special, including John Hurt who was not previously counted.
With David Tennant's Time Lord needing an extra regeneration to save himself during an earlier episode, the character's 12 lives all appear to be used up.
A show source explained to the Daily Mirror: 'The riddle of the regeneration problem, something fans have talked about for decades, will be faced head on at Christmas.'
Still guessing: Viewers have been left wondering how the next Doctor, Peter Capaldi, will join the show after 12 previous regenerations
Is this the end? The special episode saw Smith joined by Tennant, who used an extra regeneration during an earlier episode
Adding: 'There is going to be another huge cliffhanger and somehow Peter Capaldi has to join and the series has to continue.'
The Thick Of It star, Capaldi, is thought to be joining the show at Christmas, while a glimpse of him was seen for the first time at the weekend.
He was announced as the next Doctor in August after much speculation, and the next festive episode is said to be a shocking one, according to the source.
Viewers clearly aren't tired of the strange, and often confusing goings on in the show, as an impressive 10.2 million fanatics were fixed to TV screens at the weekend.
The programme went head-to-head with the ITV's The X Factor, which saw its audience figures dip, only managing to attract 7.7 million viewers - one of the lowest audiences for the show in recent years.
Doctor Who's impressive audience was the biggest overnight ratings since the programme was rebooted by the BBC in 2005.
Don't press it! 10.61 million viewers tuned in on Saturday night, to see John Hurt star as the Doctor