Kensington Palace has shared three new photographs of Prince George to celebrate his sixth birthday. Picture: Instagram

Kensington Palace has shared three new photographs of Prince George to celebrate his sixth birthday.

The future British king celebrates his birthday on Monday and his proud parents, Prince William and Duchess Catherine, have released pictures taken by the duchess to mark the occasion. 

A message on the Kensington Palace Instagram account reads: "Happy Birthday Prince George!

"The Duke and Duchess of Cambridge are very pleased to share new photographs of Prince George to mark His Royal Highness's sixth birthday.

"Thank you everyone for all your lovely messages!"

Two of the pictures show a smiling gap-toothed George in the garden at Kensington Palace wearing an English soccer jersey. In one picture he is lying down on the grass, laughing and with his eyes closed and in the other, he is smiling straight at the camera.

The caption with these photographs explained that they were "taken recently in the garden of their home at Kensington Palace by The Duchess of Cambridge". 

Another picture, taken by Duchess Catherine on holiday, shows George wearing a green polo shirt and striped shorts as he smiles for the camera. 

Prince George Alexander Louis of Cambridge was born in the Lindo Wing of St Mary's Hospital, Paddington, London, on July 22 2013.

Meanwhile, the royal - who is a big brother to Princess Charlotte, four, and Prince Louis, 14 months - recently got an early birthday treat when he received a private tennis lesson from Roger Federer. 

Federer - who has four children of his own - said of the five-year-old royal's skills: "At that stage it's all about just touch the ball, it's already good. 

"Same with my boys. I think I have a little advantage that I actually spent some time, you know, with him. I'm the only player he's ever met.  Then you have a little head start in who is your favourite player."

And he hopes he will keep up the sport.

He added: "I love to see that they're into tennis or into sports ... his mum has always enjoyed their tennis ... I hope that he'll still say the same in a few years' time."