Two Doctors get to meet in space and time

Tennant, credited with helping to revive the popular show, which was first broadcast into the nation's living rooms in November 1963, will be joined by his sidekick Billie Piper for the much-anticipated one-off reunion episode.

The pair will join the show's current leading actors, Matt Smith and Jenna-Louise Coleman, who appeared in a new series of the drama last night. Filming for the anniversary episode, which will also star the Oscar nominee John Hurt, begins this week. Smith previously told fans they would "not be disappointed" by the anniversary episode, due to air on 23 November. He said it "manages to pay homage to everything – and look forward", adding: "I read it and I clapped at the end. I think it's hilarious, it's epic and it's vast."

Details of the show were meant to be a closely guarded secret. Steven Moffat, the show's executive producer and lead writer, insisted on tight security. But news of the reunion broke on internet fan sites yesterday after subscriber copies of Doctor Who Magazine (DWM) were sent out early because of the Easter weekend. Moffat said he took extreme measures to keep the contents of the script under wraps: "This is a really good security measure – I make sure I don't get a script, because I will lose it. I forbid people to hand me one. It's on my computer under lock and key."

Piper had recently denied she was taking part in the episode when she was interviewed on The Graham Norton Show. She said: "I wasn't asked, no. I think Matt Smith may have said, in passing or in jest, it would be nice. I think maybe he said that and then it became something quite different, but no."

Doctor Who's ratings dwindled to around five million viewers in 1989, but since its return in 2005 it has been a Saturday tea-time staple, regularly getting more than 10 million viewers.

Tennant was introduced as the 10th Doctor at the end of 2005, less than a year after the show was rebooted, with Christopher Ecclestone at the helm, after a 16-year hiatus. Tennant was voted the most popular Doctor by readers of DWM in 2006.

Tennant was the first Scottish actor to play the role and last appeared in the show on 1 January 2010, after starring in three series and several specials. Since leaving, he has focused on more serious roles, particularly in the theatre, where he played a critically acclaimed Hamlet for the Royal Shakespeare Company. It was recently confirmed that he would play Richard II with the RSC in this year's winter season.

Last night's opening episode of the new series, "The Bells of St John", saw the Doctor and his companion in modern London, trying to find out what sinister forces were lurking in the capital's Wi-Fi signal. Future episodes of the show, which is filmed in Cardiff, will see the return of the Doctor's old enemy the Ice Warriors, who last appeared in 1974, when Jon Pertwee was at the helm of the Tardis.