One of the main events marking the Queen's Diamond Jubilee is set to begin at Windsor Castle.
Organisers of the Diamond Jubilee Pageant, taking place in the castle's private grounds, said the show will not be postponed by the wet weather.
Around 550 horses and more than 1,000 dancers and musicians will perform over four nights at the event, which celebrates more than 250 Commonwealth and state visits the Queen has made during her reign.
Singer Susan Boyle and entertainer Rolf Harris are among the stars to feature in the 90-minute theatrical show, which will be broadcast on ITV1 next month.
Simon Brooks Ward, who is directing and producing the pageant, said: "There is no way the show would be postponed because of poor weather.
"With over 17 countries being represented in the pageant, the world is quite literally coming to Windsor this week, and we are working with the very best suppliers and contractors to ensure our arenas as well as seating and performance areas are able to withstand the very wettest of conditions.
"We are optimistic for some brighter spells over the weekend, especially when our guest of honour, Her Majesty The Queen, arrives on Sunday for the spectacular grand finale which will see over 558 horses and over 1,200 participants perform."
The stage is a giant replica of Buckingham Palace, 45 metres (148ft) wide and 26 metres (85ft) high.
The design was chosen because the Queen's London residence is the "epicentre" of the UK, Mr Brooks Ward said.
Along with Dame Helen Mirren, who won an Oscar for her film role as the Queen, Doc Martin star Martin Clunes, The Kumars' Sanjeev Bhaskar and comedian Omid Djalili will introduce acts on stage.
Vocal quartet Il Divo, US classical violinist David Garrett, Nashville singer and banjo player Abigail Washburn, Indian folk star Raghu Dixit and South African actors from the West End musical Lion King, who will be joined by the Nairobi Chamber Chorus and Watoto Children's Choir, will also perform.