The play is due to open at the Lyttelton Theatre on 5 November
Actor Sir Michael Gambon has been forced to pull out of a star role in Alan Bennett's new play after being advised to rest due to ill health.
The 68-year-old said he is "not firing on all cylinders" and expressed his disappointment at stepping aside.
His break would have caused problems for rehearsals for The Habit of Art at London's National Theatre, which is due to open next month.
Sir Michael's role as poet WH Auden will be taken by Richard Griffiths.
The veteran star added: "The timing is very unfortunate, but I have been told to rest for a couple of weeks. This means I must let the show go on without me."
Nicholas Hytner, artistic director of the National Theatre, said he was "extremely sorry" at Sir Michael's withdrawl, and thanked Griffiths for taking over at short notice.
"We're looking forward to taking creative advantage of this unexpected twist in our own rehearsal process," added Hytner, who is directing the play.
Sir Michael, who stars as Dumbledore in the Harry Potter films, was to play alongside Alex Jennings and Frances de la Tour.
It is Bennett's first play since The History Boys in 2004, which gained success in the West End and on the Broadway stage.