Dame Helen Mirren has led tributes to the "kind and loving" actor Tim Pigott-Smith, who died on Friday aged 70.
The Oscar-winning actress worked alongside Pigott-Smith in Mourning Becomes Electra at the National Theatre.
Announcing the news, the actor's agent, John Grant, revealed he had died Friday morning, a month after collecting an OBE for services to drama.
In a statement, Mirren said: "I am so very sorry to hear of the unexpected death of my friend and colleague Tim Pigott-Smith.
"He was a kind and loving person as well as an actor with an very long, varied and successful career, brilliantly culminating in his recent triumph in King Charles III.
"I worked with him on the epic Eugene O' Neill play Mourning Becomes Electra at the National Theatre and came to love him as a person as well as an actor.
"He combined a lovely sense of humour with a great seriousness about his work and was always utterly committed.
"We will miss him."
Pigott-Smith had already filmed the forthcoming BBC2 drama King Charles III having successfully played the title role in the theatre production on Broadway and in the West End.
The play and TV drama depicts Charles as monarch after the Queen's death as he refuses to sign a controversial bill into law.
It chronicles the political chaos that follows, leading to a constitutional crisis, rioting on the streets and a tank in front of Buckingham Palace.
The play won the Olivier Award and the Critics' Circle Theatre Award for best new play and was staged in London and New York, with Pigott-Smith winning rave reviews on both sides of the Atlantic.
He was nominated for both an Olivier and a Tony Award for his performance.
He was also due to star as Willy Loman in a revival of Arthur Miller's play Death Of A Salesman, which was scheduled to open at Royal & Derngate in Northampton on April 10, before an extensive nationwide tour.
His real-life wife Pamela Miles was expected to play Willy's wife Linda but pulled out of the production after breaking a bone during rehearsals, and was replaced on the bill.
Pigott-Smith had completed production on several films, including 6 Days, a drama about the hostage situation at the Iranian embassy in London in 1980, which also stars Jamie Bell and Mark Strong, and Stephen Frears's new film Victoria And Abdul, which also stars Olivia Williams, Michael Gambon and Judi Dench and is due for UK release in September.
He is survived by his wife and their son, Tom.