Revellers will take to one of London's most famous thoroughfares today for a street party celebration in honour of the Queen's 90th birthday.
The Queen's grandson Peter Phillips has masterminded the Patron's Lunch, an open-air party being staged in The Mall, for 10,000 people, to mark the monarch's patronage of more than 600 charities and organisations.
It will bring to a close this weekend's events marking the Queen's anniversary that has already seen a St Paul's Cathedral thanksgiving service and a traditional Trooping the Colour ceremony.
The Queen, Duke of Edinburgh, Duke of Cambridge and Prince Harry will join the revellers for part of the celebration with William and Harry expected publicly to praise their grandmother.
But the event could be disrupted by the British summer as the Met Office has issued a yellow rain warning for parts of the country including London with heavy showers forecast.
Tickets for the event were priced at £150 with the majority going to organisations or charities that have the Queen as patron and the remainder sold to the general public.
Speaking on Saturday when thousands of well-wishers filled the Mall to see the Queen on Buckingham Palace's balcony, Mr Phillips described the reasons behind the celebration.
He said: "The Queen has had many celebrations in her honour over the years but there's never really been an appreciation or recognition of the number of organisations she is personally attached to through her patronage.
He added: "It was really an area of her service, to not only this country but the Commonwealth, that I felt hadn't been truly celebrated."
The Queen's grandson is a director of Sports and Entertainment Ltd, a global events agency which came up with the idea for the not-for-profit event and will receive an undisclosed fee for staging it.
Describing the moment he first told the Queen about the idea, he said: "She thought it was a fantastic idea to be able to bring all these organisations together in one go."
Mr Phillips added: "She was delighted that on this occasion she was able to give something back to all of these organisations and give them a platform to be able to celebrate their association (with her)."
The guests at the Patron's Lunch will sit at tables and dine from hampers filled with a range of treats from a can of Pimm's and a mini pork pie to two cupcakes.
And they will be entertained by a parade on The Mall where 1,600 volunteers from Patron's organisations from across the UK and Commonwealth will bring to life the decades of the Queen's reign.
The carnival-inspired procession will not only pick out key moments from the past, but will reflect a variety of categories including animals, the Armed Services, children and young people, arts and culture and others.
Each decade will be supported by 50 dancers from the Royal Academy of Dance, and accompanied by musical groups from within the Patron's organisations and beyond.