The unedited close-up image, showing a barefoot Louis grinning at the camera as he lies barefoot on a blanket on the grass, was taken in the past few days at Windsor by Kate, who announced in March she is undergoing chemotherapy after being diagnosed with cancer.

It was posted on Kensington Palace’s social media accounts, some six weeks after the furore surrounding the Mother’s Day portrait which the princess admitted editing.

William and Kate thanked royal fans for the birthday messages they received for their youngest child.

The post read: “Happy 6th Birthday, Prince Louis! Thank you for all the kind wishes today.”

The image was followed by a camera emoji and attributed to the Princess of Wales.

The Wales family have had a challenging start to the year, with the princess’s major abdominal surgery, cancer diagnosis and treatment.

The release of the portrait marks a change in the way William and Kate usually issue images for their children’s birthdays.

It was posted straight to social media on Louis’ actual birthday on Tuesday, rather than under embargo to the press the day before, as was the established pattern William and Kate have followed each year for all three of their children since they were born.

It is understood to be due to the unprecedented time the family is experiencing.

William and Kate are said to be cognisant of the fact that they have asked for privacy during the princess’ recovery, but wanted to share the photograph on social media as a way to thank those who have sent birthday wishes.

The same process is expected to be followed for Princess Charlotte’s birthday in May.