The Duke and Duchess of Cambridge are expected to make their first appearance at Royal Ascot today.
Bookmaker William Hill said it is odds on that William and Kate will join the Queen in the royal procession at the famous Berkshire racecourse, but there are also fears that the carriage parade could be scrapped.
Heavy rain caused havoc at Royal Ascot on Tuesday, with racegoers sheltering their fancy hats under umbrellas and ponchos.
But the wet weather has also softened the course - so much so that there are odds of 4/1 that the Queen's carriage will not make it down the racecourse.
"It hasn't happened for many years but the going can only get softer and we imagine that there would be nothing worse than the Queen's carriage getting stuck in the quagmire," William Hill spokesman Rupert Adams said.
Odds are 10/1 that the carriage will become immobilised if the procession takes place.
The Royal Family - especially the colour of the Queen's hat - is usually the basis for novelty bets during the five-day racing event.
The monarch surprised bookmakers by wearing a mustard yellow dress on Tuesday, despite predictions she would choose blue.
William Hill is offering bets of 5/2 that she will wear a blue hat on Wednesday.
The Queen is an avid racing fan and has attended every royal meeting at Ascot during her reign. She described her visits with her family as among her "most cherished" memories.
In a foreword in the official programme on Tuesday, she said she first went to Ascot aged 19.
One of her horses, Dartmouth, will be running in the Hardwicke Stakes on Thursday.
It is not known if William and Kate are as enthusiastic about racing as the Queen, but Kate once confessed to being allergic to horses.
She told writer Kathy Lette she did not play polo like her husband because of the allergy.
William is a keen rider and regularly plays polo. He also rode during last weekend's Trooping the Colour ceremony.
Prince Harry joined his grandmother at the races on Tuesday for the third time and presented the St James's Palace Stakes trophy.
Royal Ascot, which attracts more than 300,000 racegoers each year, is often a family affair for the Queen. On Tuesday, she was joined by all four of her children and some of her grandchildren.