Prince Louis' birthday outfit is a sweet nod to his big brother

It's Prince Louis 3rd birthday today, and his parents, the Duke and Duchess of Cambridge released a portrait to mark the occasion.

As always with her children's photos, Kate Middleton took the picture, earlier this week at Kensington Palace, before the tot left for his first day at nursery.

The picture shows a smiling Prince Louis riding a bicycle, wearing a cute outfit consisting of a navy jumper over a striped shirt, light blue shorts and blue adidas trainers.

Eagle-eyed royal fans will have noticed the blue outfit bears more than a passing appearance to the one Prince George wore in his own 3rd birthday portrait.

In the picture, taken by Matt Porteus below, Prince George was also wearing a blue outfit, in the form of shorts and a striped t-shirt.

He was playing with Kate and William's dog Lupo, who sadly died at the end of last year.

So it's very possible that Kate thought it might be a sweet nod to his older brother to dress Prince Louis in a similar outfit.

And in case you were wondering why royal boys always wear shorts, there is a reason behind it.

Dr Ed Owens, historian at the University of Lincoln and expert in the monarchy’s relationship with the media, explained why George often wore them: 'Shorts symbolise the idealisation and innocence of childhood. This boy will one day be king, so it’s key that he’s seems to be as ‘ordinary’ as possible before he assumes his role as king, which the media always characterises as burdensome and difficult. This change in portrayal from carefree child to burden adult has characterised the royal family’s image since the 1930s, and is integral to generating public empathy for them.'