The Daily Record newspaper is reporting that actress Lindsay Duncan has been cast as the Doctor's companion, named in the report as Adelaide, in the second of the forthcoming special episodes which will see out David Tennant's tenure as the Tenth Doctor.
Duncan is well known for her roles in high-profile television dramas such as GBH and Rome. She is also appearing as Margaret Thatcher in next week's much-publicised BBC Two drama Margaret, concerning the former Prime Minister's final days in power.
The paper quotes Duncan as saying: "I'm thrilled to be involved in Doctor Who. I've never done anything like this before and I'm really looking forward to working with David Tennant and the Doctor Who team."
The special's co-writer and executive producer Russell T Davies is also quoted, saying: "Lindsay is an incredibly talented actress and I've been an admirer of her work for some time. We are delighted to announce that she will be joining the team and playing the Doctor's most strong-minded companion yet."
Incidentally, in 2008 Duncan co-starred alongside forthcoming Doctor Matt Smith in a critically acclaimed theatrical production of That Face.
UPDATE - Feb 19: The BBC today confirmed Duncan's casting as Adelaide. Production on the second special - co-written by Davies and Phil Ford - is due to start this spring.