Daniel Radcliffe has described Sir Peter Shaffer as "one of the UK's greatest playwrights" as he paid tribute to the Equus author who has died aged 90.
The Harry Potter star remembered the renowned writer as "incredibly kind, generous and funny", adding that he would be "forever grateful" for receiving his blessing to portray Alan Strang in the West End and Broadway productions of his award-winning play, Equus.
Sir Peter's agent said he died at the Marymount Hospice in Ireland on Monday morning after suffering a short illness.
In a tribute posted online, Radcliffe said: "Peter Shaffer was and remains one of the UK's greatest playwrights.
"He created challenging, moving and fantastically theatrical work and was also an incredibly kind, generous and funny man."
A relative newcomer to the stage at the time, Radcliffe starred in the London (2007) and New York (2008) productions of Equus, a story of a disturbed teenager who blinds six horses.
The actor said: "In my career Equus remains one of the most important and valuable things I've ever done in terms of how much I learnt from it.
"I will forever be grateful to him for trusting me to play the character of Alan Strang at a time when I had extremely limited experience on stage. I feel very privileged to have worked with him and will miss him."
Respected as "one of the true greats of British theatre", Sir Peter wrote more than 18 plays.
Much of his working life was linked to the National Theatre, for which several of his plays were written and first produced.
Rufus Norris, director of the National Theatre, said: "Peter Shaffer was one of the great writers of his generation and the National Theatre was enormously lucky to have had such a fruitful and creative relationship with him.
"The plays he leaves behind are an enduring legacy."
A statement on the National Theatre website said: "It is with great sadness that we learn of the death of Sir Peter Shaffer, CBE. He was an extraordinary writer, closely associated with the National Theatre."
The theatre added that Amadeus will return to the Olivier stage in the first National Theatre revival since its 1979 premiere.
The official Twitter account for the Tony Awards tweeted: "Sad to learn #TonyAwards-winning author of Amadeus and Equus is gone. #RIP Peter Shaffer."
Equus and Amadeus, the two works for which Sir Peter remains best known, won Tony Awards for best play in 1975 and 1981 respectively.
Both were adapted for the big screen, with Amadeus winning eight Academy Awards at the Los Angeles ceremony in 1985.
Tim Downie, a writer and star of Channel 4 comedy Toast Of London, said on Twitter: "So saddened that the legend Sir Peter Shaffer has died."
He added: "Truly one of the greatest writers. I'll always cherish the times we spent together."
The British Film Institute tweeted: "We're saddened to hear of the loss of British playwright Sir Peter Shaffer."
Actor Julian Richings, 59, of Orphan Black fame, said of the Liverpool-born writer: "RIP Equus / Amadeus playwright Peter Shaffer. Such an inspiration to my generation - made theatre accessible, exciting."
A private funeral for Sir Peter will take place in London "shortly" and details of a memorial ceremony will be announced in due course.