Endeavour will star Shaun Evans as a younger version of the legendary detective.
Abigail Thaw has now joined the cast of the drama, The Sun reports.
"Abigail's a brilliant actress in her own right," Evans said. "It was amazing having her on set."
Thaw has previously appeared in shows including Vanity Fair, The Bill, Casualty, Love Soup, Midsomer Murders and Doctors.
Endeavour is being produced as part of the celebrations surrounding the 25th anniversary of the first episode of Morse.
> Inspector Morse prequel Endeavour to air on Masterpiece in US