Prince George
This photo dated Wednesday July 2, 2014, was taken to mark the first birthday of Prince George and shows the Prince during a visit to the Sensational Butterflies exhibition at the Natural History Museum in London, England.Photo by John Stillwell - WPA Pool /Getty Images

An official photograph of Prince George Alexander Louis walking has been released to mark his first birthday.

The image was taken by Press Association photographer John Stillwell, during Prince George's visit to the Sensational Butterflies outdoor exhibition at London's Natural History Museum.

The young royal is wearing blue and white striped dungarees by French label Petit Bateauover and a navy blue polo shirt-style top.

"Prince George was very lively and very sure of himself and confident - a very determined young boy," Stillwell said

"When you have children yourself you don't realise they grow up in such a short space of time, it's always other people saying how big they've grown.

"So having seen him soon after he was born and at his christening three months later, he's now gone from a baby at his christening to a toddler, that's the biggest change.

"And who does he look like? I'd have to say his dad, William," he added.

Kate Middleton and Prince William welcomed baby George on 22 July, in the Lindo wing of London's St Mary's Hospital.

Prince George, who is third in line to the throne, will celebrate his first birthday at home in Kensington Palace.

"The family are going to be marking it privately with close family and friends at Kensington Palace," a Kensington Palace spokeswoman said.