Russell T. Davies told BBC Breakfast that Bernard Cribbins will play the Doctor's companion in a two-part Christmas special.
"It's a long and huge farewell to David Tennant."
"There's probably a special in November, then there's a huge two-part Christmas special," he added. "It's going to be massive, I promise you."
David Tennant also told BBC Radio 1 that he is currently six weeks away from the end of filming on his final episodes of Doctor Who.
"I might have had a little cry. They were brilliant scripts and very moving. It was quite a big deal really.
"I sort of turned the phone off and made sure I could read it straight through without too much interruption."
"Filming is now always out of schedule, so I don't know if I'll be sadder on the last day or filming the final scenes. That remains to be seen.
"It's weird because it's been four years. It's been so all-consuming and so life-changing. It's been such a big thing."
Tennant said upcoming Easter special Planet Of The Dead would be a "kind of a romp" and "the last time The Doctor gets to have fun" before the dark ending.
Russell T. Davies told BBC Breakfast that Bernard Cribbins will play the Doctor's companion in a two-part Christmas special.
