Richard Griffiths, one of Britain's most celebrated character actors, has died from complications following heart surgery.
The stage and screen star, who played Uncle Vernon Dursley in the Harry Potter films, was 65.
Daniel Radcliffe, who played Harry Potter alongside him, led tributes to the actor whose "encouragement, tutelage and humour" made work "a joy".
Radcliffe, who also performed with Griffiths in the stage play Equus, said: "Richard was by my side during two of the most important moments of my career.
"In August 2000, before official production had even begun on Potter, we filmed a shot outside the Dursleys', which was my first ever shot as Harry. I was nervous and he made me feel at ease.
"Seven years later, we embarked on Equus together. It was my first time doing a play but, terrified as I was, his encouragement, tutelage and humour made it a joy.
"In fact, any room he walked into was made twice as funny and twice as clever just by his presence. I am proud to say I knew him."
Griffiths died yesterday at the University Hospital of Coventry and Warwickshire. Sir Nicholas Hytner, director of the National Theatre, said Griffiths's unexpected death would devastate his "army of friends".
Away from the Harry Potter films, Griffiths was most famous for his role as Uncle Monty in cult favourite Withnail And I. The married actor, who was born in Thornaby-on-Tees, North Yorkshire, and grew up caring for his deaf parents, was awarded an OBE for services to drama in 2007.
Richard E Grant paid tribute to his late co-star on Twitter. He wrote: "My beloved 'Uncle Monty' Richard Griffiths died last night. Chin-Chin my dear friend." A Twitter posting in the name of Paul McGann - who also starred in Withnail And I, read: "Awoke to the awful news about Richard Griffiths. A mighty tree has fallen. Heart goes out to his family and loved ones." A further tribute from @pauljmcgann read: "A brilliant, funny, principled man, and a true gent. Led the way on Withnail & I. Wore his great skill lightly and his great heart out." Further tributes from fans flooded the social networking site where "#RIPRichardGriffiths", "Withnail" and "Uncle Vernon" became trending topics.