Britain's Duchess Catherine will present a royal portrait of her to Prince William for his 30th birthday.

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.''