Horses from some of the world's most elite equestrian centres are due to perform at one of the main events marking the Queen's Diamond Jubilee.
Organisers of the Diamond Jubilee Pageant, taking place in the private grounds of Windsor Castle, said the show will not be postponed by the wet weather.
The Badminton horse trials were cancelled last month because the ground was waterlogged but an all-weather surface designed to cope with heavy rainfall will ensure the show at Windsor will go ahead.
Horses have been brought from around the globe to take part, such as from Italy's Carosello dei Carabinieri, the Royal Canadian Mounted Police, the Royal Cavalry of Oman, Russia's Kremlin Equestrian Riding School and India's Magnificent Dancing Marwaris.
Around 550 horses and more than 1,000 dancers and musicians will perform over the next four nights.
The event, which also features military displays, celebrates more than 250 Commonwealth and state visits the Queen has made during her reign.
The Queen and the Duke of Edinburgh will attend on Sunday night when singer Susan Boyle and entertainer Rolf Harris are among the stars to feature in the 90-minute theatrical show.
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.