He travels through time and space, saves the Earth, and has millions of fans all over the world. But as every "Whovian" knows, the Doctor cannot last for ever: Time Lords are able to regenerate only 12 times before they die.

Fans have always thought that the 13th doctor would be the last, thanks to a 1976 Doctor Who episode, The Deadly Assassin, featuring Tom Baker as the Doctor in his fourth incarnation, and revealing for the first time the regeneration limit. But a passing comment in a children's television programme later this month is set to rewrite history and cast the Doctor, iconic hero of the world's most successful and longest-running science fiction series, as immortal.

The moment comes in the CBBC spin-off show, The Sarah Jane Adventures, which stars former companion Elisabeth Sladen as Sarah Jane Smith. Matt Smith, who plays the current Doctor Who, guest stars in a two-part episode called The Death of the Doctor, to be screened on October 25 and 26. While the Doctor and Clyde Langer, played by Daniel Anthony, are in the process of outwitting spooky vulture undertakers the Shansheeth, Clyde asks how many times he can regenerate. The Doctor indicates that there is no limit. The action continues.

Fans of the show have been expecting an official moving of the goalposts for some time, but it was anticipated as part of the Christmas special, rather than in an after-school slot on the CBBC channel.

Back in 1976, 12 regenerations must have felt like a safely distant number to pluck from the ether. Now, however, with Smith playing the character in his 11th incarnation, circumventing the rule has begun to feel rather urgent. As JK Rowling hinted last week, once a hero has conquered the world, it is hard to put him away for good: we may also see an extension to the seven-book Harry Potter franchise, despite its very final ending and Nineteen Years Later epilogue.

And with the Doctor Who brand constantly expanding (including spin-offs that range from lunchboxes to the Doctor Who Live show currently on a 25-date, nine-city tour) it was hardly likely that the BBC was going to call time on the series.

It was a stroke of PR genius to slip the hotly anticipated fudging of the Doctor's longevity into a children's series: the episodes are now sure to draw in the viewers. However, Whovians (famously likened by Sarah Jane creator Russell T Davies to a swarm of mosquitoes) will be disappointed that there is no technical reason given for the change: it is simply stated in passing.

Whovian Simon says: "Many of us old-timers have looked forward to the story that addresses the end of the Doctor's life span. I'm gutted that it appears that something so integral to the show's long-term storyline has been passed over in this way."

The BBC would not say whether there would be any further explanation. "We never comment on future storylines for Doctor Who," said a spokeswoman.