The youngest son of Prince William and his wife Catherine turned six on Tuesday, with the princess, who is being treated for cancer, credited with taking a photo released for the occasion.

The image of a smiling Prince Louis was posted directly on the couple's social media accounts and not sent in advance to media organisations under embargo, as in previous years.

His parents, Britain's next king and queen, wrote in the post: "Happy 6th Birthday, Prince Louis! Thank you for all the kind wishes today."

The credit as the photographer was signed "The Princess of Wales" -- Kate's official title.

Its release comes after a furore last month surrounding a Mother's Day photograph of Kate and her three children that leading news agencies, including AFP, withdrew because it had been digitally altered.

That image had been intended to put an end to fevered online speculation about the princess's whereabouts and health as she recuperated from abdominal surgery in January.

But her later admission that she edited it only fuelled rumours, and soon afterwards she released a video announcing she was receiving preventative chemotherapy for an unspecified cancer.

Britain's domestic Press Association news agency described the photograph of Louis as an "unedited close-up image".