British actor Richard Griffiths, best known for his roles in 'Withnail and I' and the Harry Potter films, has died at the age of 65 after complications following heart surgery, his agent said on Friday.

Griffiths spent almost four decades in radio, film, on television and on stage, and received some of his industry's top awards for his role in Alan Bennett's play "The History Boys".

The portly actor filled the screen as the lascivious Uncle Monty in the cult 1987 film 'Withnail and I'.

But younger fans will remember him for his portrayal of a much crueler avuncular figure - Harry Potter's red-faced and bullying uncle Vernon Dursley.

Daniel Radcliffe, who played the boy wizard and performed with Griffiths in the stage play "Equus", said the veteran performer had encouraged and coached him and helped him get over his nerves.

"Richard was by my side during two of the most important moments of my career ... 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," Radcliffe said in a statement.

Griffiths' agent, Simon Beresford, described him as "a remarkable man and one of our greatest and best-loved actors". He said Griffiths died in hospital on Thursday.

The actor was born in Thornaby-on-Tees in Yorkshire, northern England, the son of a steelworker. Both his parents were deaf and he learned sign language to communicate with them.

After studying drama in Manchester, he worked in radio and theatre, building a reputation as a Shakespearean clown.

He reprised his role as teacher Hector in a film of "The History Boys" in 2006. One of his best known roles on television was a cookery-loving detective in "Pie in the Sky".

On stage, he was known for his intolerance of mobile phones ringing during performances, and halted plays several times to complain and even eject offending audience members.

Nicholas Hytner, director of Britain's National Theatre, said Griffiths' unexpected death would devastate his "army of friends".

"Richard Griffiths wasn't only one of the most loved and recognizable British actors - he was also one of the very greatest," Hytner said in a statement.

Griffiths was given an OBE in 2008 and is survived by his wife Heather.