Kate Middleton portrait: Catherine, Duchess of Cambridge, gets official portrait with mixed reviews

LONDON - The first official portrait of Catherine, the Duchess of Cambridge, has been unveiled at London's National Portrait Gallery -- but has met with a mixed reaction from art critics and the public.

See the portrait on ABCNews.com.

Award-winning artist Paul Emsley, who spent three-and-a-half months painting the Duchess's likeness, said he had tried to portray her warmth and personality in the picture.

"The Duchess explained that she would like to be portrayed naturally -- her natural self -- as opposed to her official self," he said in a video posted on the gallery's website.

"She struck me as enormously open and generous and a very warm person, so after initially feeling it was going to be an unsmiling portrait I think it was the right choice in the end to have her smiling - that is really who she is."

Both Catherine, who is expecting her first baby, and her husband Prince William, are said to be pleased with the finished painting, with the Duchess reportedly telling Emsley at a private viewing on Friday that it is "amazing... brilliant."

However, reaction from art critics and the public has been more mixed, with many taking to social media, including Twitter, to share their thoughts, claiming the picture is unflattering and ages the Duchess beyond her years.

Writing in the UK's Guardian newspaper, Charlotte Higgins claimed the painting transformed "a pretty young woman... into something unpleasant from the 'Twilight' franchise."

In the Independent, art critic Michael Glover wrote that the portrait was "catastrophic," combining "hamsterish" cheeks and "hair whose featheriness has been borrowed from an advert for shampoo."

Emsley, who was born in Glasgow but grew up in South Africa before returning to Britain in 1996, won the prestigious BP Portrait Award in 2007. He has previously painted portraits of Nelson Mandela and author VS Naipaul.

He created the work from a series of photographs taken during two sittings, at Kensington Palace in London and at his studio in South West England, in May and June last year.

"I'm always worried about the sitter -- are they cold, are they hot, are they comfortable -- and photography today is so accurate and so good that it is really so much easier just to take photographs and work from that," he said.

Catherine's portrait joins centuries-worth of official paintings and photographs of the British royal family in the NPG's collection. The duchess is a patron of the gallery.

NPG director Sandy Nairne said revealing the portrait, which he described as "a captivating contemporary image," was "an exciting moment."

In a statement, Stephen Deuchar, director of the Art Fund, said: "The unveiling of a first official portrait of a royal sitter is always an important and intriguing moment, defining and enshrining their public image in a new way."