Prince William and his wife, Catherine, released a picture of their daughter Charlotte to mark the princess's ninth birthday on Thursday.

In the photograph taken in the last few days by her mother, the Princess of Wales, Charlotte, the couple's second child, was pictured smiling happily in the garden of their home in Windsor.

"Happy 9th Birthday, Princess Charlotte!" a message on the couple's X account said. "Thank you for all of the kind messages today."

It has become tradition for Kensington Palace to release pictures taken by Catherine, a keen amateur photographer, to mark birthdays and other family occasions.

However, a picture issued this year on Mother's Day in March was withdrawn by a number of news organizations, including Reuters, the Associated Press and Getty, because it had been edited, something for which Kate later apologized.

That occurred shortly before the princess revealed she was having preventative chemotherapy after tests carried out in the wake of major abdominal surgery she underwent in January revealed that cancer had been present.

Heir to the throne William resumed official duties last month, but Kate will only return to the public spotlight when her medical team says she is well enough to do so.

King Charles returned to public-facing engagements this week for the first time since he was diagnosed with cancer in February.

WATCH | King Charles returns to public duties: 

King Charles to return to public duties next week

King Charles will return to public duties on April 30 following nearly three months of 'treatment and recuperation' for cancer, Buckingham Palace say