The BBC has commissioned a third series of Doctor Who in addition to the second series already planned.

The announcement comes days before the current series – a huge ratings success for BBC One – reaches its end.

With Christmas specials scheduled for 2005 and 2006, fans can look forward to 28 new episodes in total.

As Christopher Eccleston will not be returning for the next series, Casanova actor David Tennant has been cast as his replacement.

But Billie Piper, who plays the Doctor’s assistant Rose Tyler, will return for the whole of the second series.

Writer Russell T Davies has confirmed he will write six episodes of series two, which will see the return of classic villains the Cybermen.

“It’s a joy to write – you couldn’t have more fun,” he told the BBC Breakfast programme on Thursday.

He added that Eccleston’s departure was “a great loss”, but that a new lead actor would give the show “a new lease of life”.

The new series will begin filming in Wales in five weeks.