Artist Michael Noakes painted the veteran entertainer at his home in Wentworth, Surrey.
He said: "We worked in his house, on the Wentworth Estate, fitting in time between his numerous commitments.
"His is a curious life for a man in his eighties, when most successful people are relaxing after their working life is over.
"I thought about painting him looking more reflective, and he certainly does not spend all his time with a great smile on his face, but in the end I decided that I needed to settle on that characteristic grin."
The Royal Society of Portrait Painters annual exhibition runs from May 6 to May 21 at the Mall Galleries in central London.