Britain's Prince William is reportedly set to receive a special 30th birthday present a portrait of his wife Kate.

The Duchess of Cambridge sat for her first royal portrait after deciding it would be the perfect gift for her husband's landmark birthday in June.

The brunette posed for the painting while helicopter pilot William was on a tour of duty in the Falklands. Kate is believed to have secretly visited royal artist Nicky Philipps at her London studio on four occasions.

"Most couples wouldn't think a picture of a loved one would be a great present, but this is a big deal," a source told British newspaper The Sun. "It's the first portrait Kate has sat for, which is a huge thing for the royals."

William's father Prince Charles is believed to have paid for the 25,000 portrait.

Artist Nicky who has previously painted William with his brother Prince Harry has struggled to perfect Kate's portrait because she is so "too beautiful".

"Good-looking people are always difficult because they have symmetrical faces," she explained. "It is unlikely you will do them justice."

Kate is expected to present the future king with the piece of art at his birthday party, which is set to be a lavish affair.

It has been rumoured Kate will seek advice from her sister Pippa Middleton - who is a party planner - to make sure the prince's birthday is one to remember.

William turns 30 on June 21, while Kate celebrated her 30th birthday in January by attending the premiere of Steven Spielberg's movie War Horse in London.

Kate and William have a deep passion for art, having both studied history of art at St. Andrew's University in Scotland.

Kate is a charity patron of the National Portrait Gallery in the UK. It was recently confirmed that she has agreed to a request to pose for a painting which will be displayed in the gallery.