Duncan will next be seen as Margaret Thatcher in a BBC Two drama
Lindsay Duncan is to guest-star as an assistant in Doctor Who in the programme's second special of 2009.
Duncan will play Adelaide, the Doctor's cleverest and most strong-minded companion yet in the unnamed episode.
The actress, who has starred in Rome and GBH, said "I'm thrilled to be involved in Doctor Who. I've never done anything like this before."
There are four Doctor Who specials being filmed in 2009 with the first - Planet of the Dead - airing at Easter.
Russell T Davies and Phil Ford are joint writers of the second special and filming will start on the episode in Spring.
David Tennant stars in all four programmes, concluding his run as the Doctor before 26-year-old Matt Smith takes over the role in 2010.