Royal Mail is celebrating Prince William and Kate Middleton's wedding with a new set of stamps.

The collectibles will feature William and Kate's official engagement portraits taken by fashion and celebrity photographer Mario Testino.

His casual picture of the couple will go on a first-class stamp, with the more formal image on a £1.10 stamp.

They will be released on the Queen's birthday, April 21, and have been approved by the monarch, with William also involved in the process.

The commemorative stamps are the first to mark the wedding of one of the sovereign's grandchildren.

Royal Mail director Stephen Agar said: "We are honoured to be celebrating this very special and historic occasion. We believe that that this set of stamps will be very popular with people both in the UK and around the world."

William has featured on Royal Mail stamps twice before: to mark the Queen Mother's 100th birthday on August 4, 2000, and three years later to mark his 21st birthday.

The new stamps will come in a sheet of four: two first-class and two £1.10 stamps.

Post Office branches will sell them from April 21 but the stamps can be ordered from Royal Mail's website from April 7.

The marriage of William's parents, the Prince and Princess of Wales, in 1981 was marked by two Royal Mail stamps - a 14p first-class stamp and a 25p one for overseas mail.

The first-class stamp marking William and Miss Middleton's wedding will be priced at 46p - as the cost of a first-class stamp increases by 5p from April 4.

The new £1.10 royal wedding stamp covers the postage of letters and cards, weighing up to 20g, sent worldwide.