Paul Mealor said he has written the tune Sleep On because it seemed a natural progression to accompany the next stage of their married life.
Westenra has included the song on a new album of lullabies and a copy is to be sent to the couple to help them through any sleepless nights with their new arrival, due in July, and she has dedicated the album to the royal baby.
Mealor, who also wrote the chart-topping Wherever You Are for the Military Wives choir, was commissioned to write Ubi Caritas for the wedding two years ago and was regarded as one of the musical highlights.
The 37-year-old said: “After writing music for the Royal Wedding it seemed natural to provide the musical accompaniment for the next stage of the royal couple’s journey together.”
Westenra’s new album, called Hushabye, is designed to calm parents and children alike with calming tunes such as Hushabye Mountain and Twinkle Twinkle Little Star and is being released in June.
Westenra, who has performed for the Prince of Wales, said: “Paul and I got together to work on a song called Sleep On in honour of William and Kate’s new arrival as it’s such a joyous occasion which we can all celebrate.”