Image copyright 2016 Nick Knight Image caption In the photo, Prince Charles appears to smile at the Queen, who looks directly at the camera

A photograph of the Queen and the Prince of Wales has been released to mark the end of her 90th birthday year.

The photo shows the Queen with her son Prince Charles in the White Drawing Room at Windsor Castle.

Previously unpublished, it was taken before the final night of celebrations at the Royal Windsor Horse Show in May.

Fashion photographer Nick Knight, who took the photo, said he had wanted to create something that showed "warmth, humanity... strength and tradition".

The Queen, who last year became Britain's longest-reigning monarch, turned 90 years old in April.

In pictures: Queen at 90 - in 90 images

She celebrated the occasion with a series of events, including a private black tie banquet at Windsor Castle with her friends and loved ones.

Analysis

By Peter Hunt, BBC News royal correspondent

The most arresting images can be the ones that leave you wanting to know more.

In this instance, what was the wisecrack uttered by the son that made the mother want to laugh as the camera captured them together?

On a day in May devoted to horses this photographic assignment was over in minutes.

The significance of the image is obvious. This is a 90-year-old monarch with her heir by her side. As things stand, the future of the House of Windsor is secure.

One of the most recognisable women on the planet has sat, countless times, for photographs and portraits.

She never comments on the finished results. Others aren't so reserved.

An oil painting by Lucien Freud was said by one art critic to have given the Queen six o'clock shadow and made her look more like a rugby player than a monarch.

This latest addition to the genre prompts us to wonder about the nature of the joke cracked by Prince Charles; a joke the four people in the room when the shutter clicked may well now have forgotten.

Read more from Peter Hunt

Queen Elizabeth's official birthday was marked in June with a service of thanksgiving and a picnic on The Mall, in central London, for 10,000 guests.

In a speech at the event, the Queen had joked: "How I will feel if people are still singing Happy Birthday in December remains to be seen."

About the newly-released portrait, Mr Knight said: "It was a great pleasure and an honour to photograph Her Majesty The Queen and HRH The Prince of Wales at Windsor Castle.

"I wanted to create a modern portrait that showed warmth and humanity as well as strength and tradition."