The 29-year-old who is best known for her work in films Widows, The Great Gatsby and The Man From Uncle, joins the already announced cast of Imelda Staunton as Queen Elizabeth, Lesley Manville as Princess Margaret and Jonathan Pryce as the Duke of Edinburgh.
The show’s final seasons featuring Elizabeth as the Princess of Wales, will depict the royals in the 1990s - one of the family’s most turbulent and scandalous periods.
Concluding in the early 2000s it is believed the show will cover Diana’s divorce from Prince Charles as well as her tragic death.
“As we started to discuss the storylines for [Season 5], it soon became clear that in order to do justice to the richness and complexity of the story, we should go back to the original plan and do six seasons,” the show’s creator Peter Morgan said in a statement.
“To be clear, [Season 6] will not bring us any closer to present day — it will simply enable us to cover the same period in greater detail.”