Britain's Prince William and Kate Middleton on Sunday released two new official photographs of their newborn baby Prince Louis, one of which shows him being cuddled by his elder sister Princess Charlotte.

Louis Arthur Charles, who is fifth in line to the British throne, was born on April 23 and is William and Kate's third child.

The Kensington Palace released two pictures taken by Kate, the Duchess of Cambridge, documenting the early days of Britain's newest royal.

One of the pictures shows a sleeping Louis being cuddled and kissed by his three-year-old sister, Charlotte, and the other a solo of him which was snapped when he was just three days old.

Prince William and Kate were "very pleased" to share the photographs, the palace said in a statement.

"Their Royal Highnesses would like to thank members of the public for all of the kind messages they have received following the birth of Louis, and for Charlotte's third birthday," the statement said.

Charlotte turned three on May 2 and bears a strong resemblance to her new-born brother when she was a baby.

Due to a 2013 change in the law, Charlotte is the first British princess for whom the arrival of a younger brother does not mean being demoted down the line of succession.

Their older brother, four-year-old Prince George, who will turn five in July, does not appear in the latest photographs. He was last seen in public outside St. Mary's Hospital when he and Charlotte were taken by their father, the Duke of Cambridge, to meet the latest addition to their family.

Kate has released a series of photographs snapped by her to mark milestones in her children's lives, including the official photographs of her new-born daughter, and George and Charlotte's first days at nursery school.