The Royal Mail stamps for the wedding of Prince William to Catherine Middleton will be issued on the Queen's birthday, April 21.

A four-stamp miniature sheet features two portraits by Princess Diana's favourite photographer Mario Testino. They are official engagement photographs. The formal image (on the £1.10) was taken in the Council Chamber of St James’s Palace, while the informal image (on the 1st class) shows the couple in the Palace’s Cornwall room.

Each of the stamps bears the date of the wedding as 29.04.11. The grey border bears the words 'The Royal Wedding HRH Prince William of Wales & Catherine Middleton' in Welsh and English.

Prince William and the Queen gave approval for the stamps.

This is the third time the Prince has appeared on a Royal Mail stamp. He appeared on a miniature sheet for the Queen Mother's 100th birthday in 2000, and on a set for his 21st birthday in 2003.

Royal Mail promises a souvenir cover and a coin cover. A 'commemorative document' will mark the wedding day itself, with images of the day and a miniature sheet postmarked onto the document.