The first official painting of the Duchess of Cambridge has been unveiled this morning and provoked a dismayed reaction.

It was painted by the artist Paul Emsley and took several months using a technique of building thin layers of oil and glazes on canvas.

Kate Middleton sat for the artist twice. Paul Emsley also worked from a series of photographs he took of Catherine.

The portrait is due to go on show on Friday afternoon at London’s National Portrait Gallery, of which the duchess is patron.

Paul Emsley was chosen to paint the portrait by the gallery’s director, Sandy Nairne. Kate Middleton, who studied history of art at St Andrews University, was also involved in the selection process.

Kate Middleton, 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.

Paul 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 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 Paul Emsley won the BP Portrait Award for his depiction of fellow artist, Michael Simpson.

Paul 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.”

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

“I think it was the right choice in the end to have her smiling. That’s really who she is, I think,” he said.

He also knew he wanted to make a feature of Kate Middleton’s hair.

“Everyone, I think, recognizes her partly through her lovely hair,” he said.

“I’ve altered the color of the eyes slightly to match the color 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.

The first official painting of Kate Middleton has been unveiled this morning at London’s National Portrait Gallery

Sandy Nairne said it was an “exciting moment” to display the first commissioned public portrait of the duchess.

He added that she was grateful to the duchess for giving up her time for the sittings and to Paul Emsley for creating a “captivating contemporary image”.

The portrait, called HRH The Duchess of Cambridge, will be displayed as part of the National Portrait Gallery’s Contemporary Collections.

Critics slammed the “rotten” picture by Paul Emsley after it was unveiled at London’s National Portrait Gallery, saying it looked “nothing like Kate in real life”.

Sunday Times art critic Waldemar Januszczak called the portrait “pretty ordinary… He made her look older than she is and her eyes don’t sparkle in the way that they do and there’s something rather dour about the face”.

Paul Emsley- who is famous for dividing opinion with his extraordinarily realistic, almost “photographic” style – was commissioned by the gallery after Kate Middleton chose him from a shortlist.

Kate Middleton and husband Prince William visited the gallery early this morning ahead of a public viewing, and she was said to be “thrilled” with the results of her latest royal rite of passage.

Since joining the Royal Family Kate Middleton has become one of the most scrutinized women on the planet – and her portrait is sure to cause controversy.

Robin Simon, editor of the British Art Journal, said today: “Fortunately, the Duchess of Cambridge looks nothing like this in real life.

“I’m really sad to say this is a rotten portrait.”

The head and shoulders portrait shows the Duchess, who celebrated her 31st birthday on Wednesday, staring directly out of the picture with a “serious but serene” expression on her face.

Kate Middleton is wearing a bottle-green pussy bow blouse imagined by the artist and a pair of earrings that were a wedding gift.

Paul Emsley, who has also painted Nelson Mandela, was apparently keen to capture her “charm, serenity and intelligence” as well as her “sparkling green eyes”.

At first glance the Duchess, who celebrated her 31st birthday this week, looks far older than her years.

There are shadows and creases under her eyes while her famous brunette locks have a strangely coppery tinge to them.

Her mouth is set in a grimace rather than the promised enigmatic smile.

Paul Emsley had said: “The Duchess explained to me that she would like to be portrayed naturally – her natural self as opposed to her official self. She struck me as enormously open and generous and a very warm person.

“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.”

Kate, Prince William and the entire Middleton family – Carole, Michael, James and Pippa – went for a private viewing of the portrait this morning before it was publicly unveiled.

The Duke and Duchess of Cambridge were first to congratulate the artist on his work.

“I thought it was brilliant,” said Kate Middleton, who showed no sign of her 12-week bump in a deep red Whistles dress.

“It’s just amazing. Absolutely brilliant.”

Kate Middleton spent 10 minutes studying the portrait before gesturing with her hands, indicating that she was particularly pleased with the way he had captured her around her mouth and nose.

“Thank you,” added the polite royal, who met the 65-year-old artist with his wife Susanne and one of his daughters, Maria, in a separate room after viewing the portrait privately away from him.

“I had such a fun day at the studio,” Kate Middleton told Maria.

Prince William, wearing a dark suit, was equally enthusiastic.

“It’s beautiful. Absolutely beautiful,” he told Paul Emsley.

The Duchess is patron of the NPG in London, which has been keen to commission a picture of her for some time.

The gallery drew up a shortlist of four potential painters and Kate Middleton, who graduated from St Andrew’s University with a 2:1 in history of art, personally plumped for Paul Emsley.

Her portrait is situated in Room 37, next to a famous installation showing a video of a sleeping semi-naked David Beckham. It has been donated to the gallery by supporter Sir Hugh Legatt.

It is the same ground-floor gallery that holds a picture in oils of her husband Prince William and his younger brother, Prince Harry, which was unveiled there in 2010.

The two princes posed in full military regalia for artist Nicky Philipps for their first official depiction.

Prince Harry later joked that he was “a little too ginger and William had way too much hair” but declared himself otherwise pretty pleased with the result.

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