Image copyright National Portrait Gallery Image caption The painting was commissioned by the National Portrait Gallery

The first official painting of the Duchess of Cambridge has been unveiled to mixed reviews from critics.

Paul Emsley's work took several months using a technique of building thin layers of oil and glazes on canvas.

The artist had two sittings with the duchess and also worked from a series of photographs he took.

The portrait is now on display at London's National Portrait Gallery, which commissioned the work. The duchess is the gallery's patron.

Mr Emsley was chosen by the gallery's director, Sandy Nairne. Catherine, who studied history of art at St Andrews University, was also involved in the selection process.

The duchess, 31, whose pregnancy was announced by the Royal Family last month, sat for the artist in May 2012 at his studio, and again in June at Kensington Palace.

'Light and shadow'

Mr Emsley said she had expressed a desire to be portrayed as her natural self, rather than her official self.

The artist described his work as simple.

I think it was the right choice in the end to have her smiling Paul Emsley, Artist

"I don't have lots of things in the background. I do like large faces, I find them strong and contemporary," he said.

"I'm interested in the landscape of the face, the way in which light and shadow fall across the forms. That's really my subject matter.

"To have anything else in there is really just an interference."

The artist's other subjects have included the former President of South Africa, Nelson Mandela, and the author V S Naipaul. In 2007 he won the BP Portrait Award for his depiction of fellow artist, Michael Simpson.

Mr Emsley said it was easier to work from photographs.

"I'm always worried about the sitter - are they cold, are they hot, are they comfortable?" he said.

"Photography today is so accurate and so good that it's really so much easier just to take photographs and work from them."

Smiling portrait

Originally, Mr Emsley had planned to produce an unsmiling portrait. But on meeting the duchess he changed his mind.

"It was the right choice in the end to have her smiling. That's really who she is," he said.

He also knew he wanted to make a feature of Catherine's hair.

"Everyone, I think, recognises her partly through her lovely hair," he said.

Media playback is unsupported on your device Media caption Sunday Times Art Critic Waldemar Januszczak says the work is "disappointing"

"I've altered the colour of the eyes slightly to match the colour of the blouse and the blue background."

The painting took three-and-a-half months to complete and was presented to the portrait gallery's trustees in November.

Waldemar Januszczak, art critic for the Sunday Times, said he was not impressed by the painting.

"I was disappointed, to be honest. I have been waiting for it, like everybody else, with great expectation because the Duchess of Cambridge is someone who we know likes art and was presumably going to be an enlightened patron," he said.

"But I think she's been let down really by the picture. In the end it's yet another pretty ordinary painting of a royal of the sort that we've been really churning out for the last few hundred years in Britain."

Paul Emsley was asked how he felt about some art critics' less than flattering comments about his work.

He told BBC News: "I tried my best. Inevitably it's not to everyone's taste, I understand that.

"I'm developed as an artist to accept the fact there are different points of view, but I believe in what I do and I have to stay with that."

'Exciting moment'

Alastair Adams, president of the Royal Society of Portrait Painters, said Mr Emsley had showed the natural, not official, side of the duchess.

"It's very human - when you look at it, the full face is in front of you, you look straight into the eyes and face," he said.

"There are no airs and graces, there's no background context to allude to success or power - it's very much on a level of one to one with the viewer.

"It's quite natural, it's open, it's straightforward and very pure - it's immediate and not overly sentimental."

The portrait was given to the gallery by Sir Hugh Leggatt in memory of Sir Denis Mahon, with support from the Art Fund.

Art Fund director Stephen Deuchar 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."

Mr Nairne said it was an "exciting moment" to display the first commissioned public portrait of the duchess.

He added that he was grateful to the duchess for giving up her time for the sittings and to Mr Emsley for creating a "captivating contemporary image".

The portrait, called HRH The Duchess of Cambridge, is on display as part of the National Portrait Gallery's Contemporary Collections.