Tennant will co-star alongside Elisabeth Sladen, who plays Sarah Jane Smith

David Tennant is to appear as the Time Lord in Doctor Who spin-off The Sarah Jane Adventures, the BBC has announced.

Tennant, 38, will appear over two episodes of a new 12-part series of the CBBC show, which starts in September.

Russell T Davies, executive producer of The Sarah Jane Adventures and Doctor Who, said it would be a "full on appearance" and "not just a cameo".

The show follows the adventures of the investigative journalist Sarah Jane Smith - a former Doctor's companion.

Smith is played by actress Elisabeth Sladen, 61, who also played the character in Doctor Who opposite Jon Pertwee and Tom Baker between 1973 and 1976.

'Special treat'

Sladen said she was "absolutely over the moon" to be co-starring alongside Tennant.

"It's fantastic news that Sarah Jane is going to spend some time working with the Doctor again and is testament to just how successful this CBBC series is."

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Davies said: "Viewers thought they may have to wait until November for the next full episode of Doctor Who, but this is an extra special treat.

"And it's not just a cameo from David.

"This is a full-on appearance for the Doctor as he and Sarah Jane face their biggest threat ever."

The third series of The Sarah Jane Adventures will feature monsters who want to infect earth and a "living painting".

Tennant is being replaced as Doctor Who by actor Matt Smith, 26, who will appear as the 11th Doctor next year.

Tennant will firstly appear in three specials, starting with The Waters of Mars in November.

Meanwhile, Doctor Who may be heading for the big screen after a spokeswoman for BBC Films confirmed that "a script is in development".

BBC entertainment correspondent Lizo Mzimba said there were no guarantees a film would be made and that, if the project went into full production, a release would be a long time away.