News of the transfer comes just before the delayed press performance of McKellen's Lear, which will be shown to the critics as a matinee, followed by an evening performance of The Seagull, in which McKellen plays Sorin.
The Shakespeare and Chekhov plays will run in repertory at Stratford-upon-Avon's Courtyard Theatre until June 23.
The RSC made the decision to postpone the original April 3 press night of King Lear after Frances Barber, who plays Goneril, sustained a knee injury in a bicycle accident.
The injury prevented Barber from performing in Lear and rehearsing for the leading role of Arkadina in The Seagull, the second play in the repertory, which begins previews April 17.
The delayed press performances allowed the ensemble company time to rehearse and perform The Seagull in repertory with Lear following Barber's recovery. Melanie Jessop stepped in for Barber for the preview performances of both plays. McKellen's Lear rounds off the RSC's Complete Works season and marks the actor's return to the company after 17 years. He will alternate in the role of Sorin with actor William Gaunt.
Gaunt (credits include Humble Boy and The Cherry Orchard for the National) makes his RSC debut in the role-sharing production.
Following the run at the Courtyard, both productions will embark on an international tour taking in Singapore, Australia, New Zealand and arriving in the U.S. on Sept. 6 at New York's Harvey Theatre.
The tour then moves on to the Guthrie Theatre, MN, beginning Oct 5 and Los Angeles' Royce Hall beginning Oct. 19.
When the productions arrives in London, they will be hosted by the same theatre that housed Nunn's long-running production of Cats. The New London is owned by that show's composer, Andrew Lloyd Webber, whose Really Useful Group runs the venue.
The ensemble cast for The Seagull includes Romola Garai and, for Lear only, Sylvester McCoy.
Garai, best known for such screen work as "Inside I'm Dancing" and "Vanity Fair," plays Cordelia in Lear and Nina in The Seagull. The plays represent her RSC debut, which is also true of her fellow rising star, drama student Richard Goulding, who plays Konstantin in The Seagull.
Barber was last seen in the West End in One Flew Over the Cuckoo's Nest with Christian Slater. Other credits include Cleopatra in Antony and Cleopatra and Aladdin, with McKellen's Widow Twanky.
McCoy, who last appeared with the RSC in the company's 2001 production of The Lion, the Witch and the Wardrobe, plays The Fool in Lear. He is best known for his television appearances as "Dr. Who."
Speaking about the two productions, Nunn, who was the RSC's artistic director from 1968 - 1986 said, "For many years now I have been hoping to return to my theatrical roots in the RSC, and for even longer, I have been looking forward to the fulfilment of the vow Ian McKellen and I made that one day we would do King Lear together. Add to that the fact that we both believe in ensemble work playing in repertoire, and you have the background of this exciting plan to perform these two plays with the same cast in Stratford-upon-Avon."
For more on King Lear and The Seagull, call (0)870 7845 0512.