The Duchess of Cambridge posed for the £25,000 canvas artwork – which was painted by royal artist Nicky Philipps, who has also painted William and Prince Harry – on four separate occasions while her husband was away on a six-week tour of duty in the Falklands earlier this year.
A source told The Sun newspaper: "Most couples wouldn’t think a picture of a loved one would be a great present, but this is a big deal.
“It’s the first portrait Kate has sat for, which is a huge thing for the royals.”
William's father, Prince Charles, is thought to be paying for the present, and the duchess is said to have plenty more gifts "up her sleeve" for her husband's birthday on June 21.
A royal spokesperson confirmed a “privately commissioned painting” of Catherine – who was known as Kate Middleton before she married William in April 2011 – has been completed.
But Nicky revealed the duchess was "difficult" to paint because she was so "good looking".
She explained: "Good-looking people are always difficult because they have symmetrical faces.
“It is unlikely you will do them justice.”
He added: "My favourite photograph was the one where they were sitting on the steps in the throne room with all of the bridesmaids around them and we had literally minutes to do that. We had almost run out of time but everyone was organised and we were ready to do it and we had bribes of jelly beans for the kids and within three minutes we were able to get everyone together and take the picture. The personalities of the individuals really shine through because we didn't have time to stage anything and I think it's wonderful."
Hugo Burnand has teamed up with Schweppes to find the nation's social monarchs: the kings and queens of entertaining who are celebrating the Diamond Jubilee in style. To nominate yourself or a friend as a social monarch, or to pick up some great tips for your Jubilee party visit www.schweppesjubilee.co.uk.