Josh had long been rumoured for the role and confirming the news, he said: “I am thrilled to be joining The Firm for the next instalment of ‘The Crown’.
“Seasons three and four will follow some of the most turbulent events in the Prince of Wales’ life and our national story and I’m excited to be bringing to life the man in the midst of it all.
“I’m very aware it’s a formidably talented family to be joining, but reliably informed I have the ears for the part and will fit right in.”
Of her role, Marion - whose previous credits include TV show ‘Him And Her’ and film ‘Mr Turner’ - added: “Wonderful to be joining ‘The Crown’. It’s a brilliant show and we have a tough act to follow but what a gift to be playing the fascinating and greatly loved Queen Mother.
“Thrilled to be on board and working with such a top-notch team.”
Series three will air in 2019 and begin where the last left off, with the Queen at the age of 29.
There will be a total of six seasons, Netflix’s Ted Sarandos has previously explained, and the aim is for them to take the story up present day.