The Booker Prize winner said the pair were like “good secretaries to each other” as they worked through lockdown swapping drafts of the show based on her novel The Mirror and the Light.
The show will get its world premiere at the Gielgud Theatre from September 23 with Miles again in the role of Thomas Cromwell – whose rise and fall from power at the court of Henry VIII has been told in the three books which have been adapted for both stage and screen.
Mantel said she started working with Miles as a writer when the play first went to Broadway and they would swap ideas for minor changes to the script.
She said: “Ben and I would go to our separate places home from rehearsals and started a very swift email process of knocking together things that we could take into the cast the next day, whether it was something to be patched over or actually an opportunity we saw to push something home a little more clearly.
“So we got used to that swift idea exchange and I really admired the fact that there is Ben right in the process, he is in every scene, the whole thing spins around him, so then I thought when it comes to the third play who would know better?”
Miles said the pair carried on “a back and forth correspondence” over email as they co-wrote the third play “pretty much scene by scene” though they were able to get together for a read-through in the brief period when lockdown lifted.
Mantel said: “One of the things that drove us along was knowing that one of the things people were really missing was being in a crowd or an audience and being able to give a collective response and I think we knew the appetite would be there for the show so our appetite was there.”
Miles, who also starred in royal drama The Crown, had never written anything before and said: “It is a bit of a hard act to follow as a writer of plays this whole collaboration and project for obvious reasons but I certainly don’t see myself stopping writing after this but what it will be I don’t know”.
The new stage show will reunite Miles with co-star Nathaniel Parker, playing Henry VIII, and director Jeremy Herrin.
Tickets go on sale on Thursday May 27 and you sign up for more information and priority booking from May 24 at themirrorandthelight.co.uk.