Britain's Prince William said on Saturday that he knew the day would come when his "Grannie" Queen Elizabeth II would be no more but that it would take some time for the reality to sink in, as he reflected upon how the world had lost an extraordinary leader .

In his first statement as the Prince of Wales and heir to the British throne, posted on Instagram, the 40-year-old said he would honour the memory of the late monarch by supporting his father, the new monarch.

I knew this day would come, but it will be some time before the reality of life without Grannie will truly feel real, William said in his short statement.

On Thursday, the world lost an extraordinary leader, whose commitment to the country, the Realms and the Commonwealth was absolute I, however, have lost a grandmother. And while I will grieve her loss, I also feel incredibly grateful. I have had the benefit of The Queen's wisdom and reassurance into my fifth decade, he said.

My wife has had twenty years of her guidance and support. My three children have got to spend holidays with her and create memories that will last their whole lives. She was by my side at my happiest moments. And she was by my side during the saddest days of my life, he reflected.

Thanking his late grandmother for the kindness she showed him and his family, the heir who is now also Duke of Cornwall expressed his gratitude on behalf of my generation for providing an example of service and dignity in public life that was from a different age, but always relevant to us all .

My grandmother famously said that grief was the price we pay for love. All of the sadness we will feel in the coming weeks will be testament to the love we felt for our extraordinary Queen. I will honour her memory by supporting my father, The King, in every way I can, he concluded.

Soon after he was joined by his wife, Catherine Princess of Wales, and brother Harry and his wife, Meghan Markle, as they viewed the floral tributes left by members of the public at the gates of Windsor Castle the Queen's royal residence in Berkshire, south-east England.

In what is seen as a moment of royal unity amid reports of tension between the two brothers, the foursome shook hands and spoke to members of the public gathered at the castle to pay their respects in the wake of the Queen's death.

Queen Elizabeth II, the UK's longest-serving monarch, died on Thursday at Balmoral Castle in Scotland after reigning for 70 years. She was 96.

Harry and Meghan join Prince and Princess of Wales at Windsor Castle to greet crowds

Prince Harry and his wife Meghan joined Prince William and wife Kate to examine the flowers and tributes left outside Windsor Castle on Saturday following the death of Queen Elizabeth II.

This is the first time the two couples have appeared together since Harry and Meghan moved to the United States, leaving their royal duties behind.

A spokesman for Prince William said that he had invited Harry and Meghan to join him and Kate as they examined the tributes and greeted the crowds who were waiting outside of the entrance to Windsor Castle.

Witnesses report that William said the days since the queen's death had been "surreal".

"We all thought she was invincible," he said.

With inputs from Deutsche Welle Newsfeed