Britain's Prince William and wife Kate Middleton are celebrating their oldest son's birthday.

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Prince George turned 11 on Monday, and his mother, formally known as Princess Catherine, captured the moment in a black-and-white photograph, which was shared on the couple's Instagram account.

"Wishing Prince George a very happy 11th birthday today!" the caption reads.

The photo amassed more than 8,700 comments within a few hours, with several commenting about how fast time has flown.

"He's the splitting image of his father, honestly," one commenter said. "Happy Birthday George! And how is he 11 already?"

This photo is the latest portrait taken by the princess of Wales to commemorate her children's birthdays. She shared a photo of Prince Louis in April for his sixth birthday, and a photo of Princess Charlotte in May for her 9th birthday.

Middleton, who is fighting cancer, has largely been out of the public eye recently, apart from a recent appearance at Wimbledon.