document: The painting was revealed by the veteran broadcaster and naturalist at New Walk Museum, in Leicester.
  He grew up in the city and volunteered at the museum as a teenager.
  The artist - Bryan Organ - has also painted Sir David's brother Lord Attenborough, Prince Charles and former prime minister Harold Macmillan.
  The portrait was commissioned by the City of Leicester Museums Trust and will complement the portrait of Lord Attenborough, the acclaimed actor and film director, which also hangs in New Walk Museum.
  More on this and other Leicestershire stories
  Sir David said it was "an honour" to sit for Mr Organ.
  When asked what he made of the portrait, he said: "I haven't had the chance to have a look at it - I mean, I've only seen it obliquely. As far as I can see, it's very nice.
  "It was great fun sitting with Bryan, the only problem was the painting got in the way."
  The Attenborough brothers grew up in the city where their father was principal of the then University College Leicester.
  Tributes have been paid to Sir David throughout the year since he turned 90 in May.
summary: A portrait of Sir David Attenborough has been unveiled to commemorate his 90th birthday - in a museum where he once volunteered.