The Duchess of Cambridge reportedly described it as "amazing." Prince William liked it too, calling it "absolutely beautiful."
The portrait was created by award-winning artist Paul Emsley and is set against his trademark dark background. The 31-year-old royal is depicted with her lips pursed into a wry smile.
Following the duchess' suggestion, the portrait depicts her natural, not official, persona. She was joined by her husband for a private viewing of the painting Friday morning before going on display to the public in the afternoon.
Shortly after the portrait was unveiled, the critics came out of the woodwork.
"It's a great, great opportunity missed," British Art Journal editor Robin Simon said. "The best thing you can say about it is that she doesn't actually look like that."
In a telephone interview with the Associated Press, Simon said that Kate's nose was too large and that the painting drained the duchess of her sparkle.
Kate "transmits a sense of joie-de-vivre," he said. "This is dead, dead, dead."
Guardian arts writer Charlotte Higgins picked up on that theme, saying the portrait had a "sepulchral gloom" about it.
Emsley also received praise for his work.
"I liked it, very much so," said Richard Stone, who has frequently painted members of the royal family. "So often with official portraits they can be rather stiff and starchy, but this has a lovely informality about it, and a warmth to it."
The Associated Press contributed to this report.