The author visited the set of Johnny and the Bomb

Actors Zoe Wanamaker, Frank Finlay and Keith Barron will star in the three-part CBBC adaptation to be shown on BBC One in 2006.

The third title in Pratchett's Johnny Maxwell trilogy is being filmed by Ballykissangel director Dermot Boyd.

The story follows the adventures of a mysterious bag lady who holds the key to time-travelling.

"I'm thrilled that Terry Pratchett has agreed to let CBBC adapt his best-selling and much-loved Johnny and the Bomb for television," said CBBC controller Alison Sharman, who commissioned the series.

"It's a hugely entertaining story, and has attracted a magnificent cast."

Radio drama

The story has been adapted for television by Peter Tabern, who won an International Emmy and the Bafta children's writer award for CBBC's Stig of the Dump in 2001.

The drama is being produced for the BBC by Childsplay Television and is being filmed in South Wales.

Pratchett has visited the set to see for himself how filming on the drama is progressing.

There was a surge of interest in Pratchett's works after BBC Radio 4 serialised his first Discworld novel, The Colour of Magic in 1983.

In addition to the long-running Discworld series, which is set on a flat planet, Pratchett also writes for children and young adults.

Johnny and the Dead, the second title in the Johnny Maxwell books was made into an ITV drama in 1995.