Ophelia Lovibond will play Carrie Symonds in the upcoming drama about Boris Johnson's handling of the coronavirus crisis.
It has already been announced that Sir Kenneth Branagh will play the Prime Minister in Sky series This Sceptred Isle.
A first-look image of Branagh in the role showed the 60-year-old actor with blond tousled hair as he stares, frowning, into the distance.
Lovibond, who is also known for her roles in Guardians Of The Galaxy and comedy series Feel Good, will play the Prime Minister's fiancee.
Simon Paisley Day, who recently played Douglas Carswell in the TV drama Brexit, starring Benedict Cumberbatch, will play Dominic Cummings.
The five-part drama will chart the events during the first wave of the global pandemic and is based on the first-hand testimony of people from Downing Street, the Department of Health, the Scientific Advisory Group for Emergencies (Sage), care homes and hospitals, according to the broadcaster.
Filming has begun for the series, which is being directed by Michael Winterbottom.
Tim Shipman, political editor of the Sunday Times, is acting as a consultant.
The drama will begin with Mr Johnson's appointment as Prime Minister and will recount the first coronavirus cases in the UK.
It will tell the story of the 56-year-old premier falling ill with the virus, as well as the arrival of his son, Wilfred.
This Sceptred Isle will also focus on the impact of the pandemic on the country as a whole.
The series will air in the autumn of next year on Sky Atlantic and NOW TV.