Visitors to the famous Berkshire racecourse will have to contend with showers as well as the new stricter dress code, which threaten to rain on the parade of traditionally colourful and eye-catching outfits.
The Met Office issued a yellow weather warning for rain for the West Midlands, East Midlands, east of England, south-west and south-east and London while parts of Scotland were also due to see outbreaks during the day.
The alert – which is the lowest of three levels of severity – is valid from 3pm today until noon tomorrow and warns of standing water on roads as well as the chance of localised flooding.
A thunderstorm is also forecast for 2pm, when the Queen travels down the racecourse in a horse-drawn carriage procession.
Matt Dobson, a forecaster with Press Association's MeteoGroup, said: "There will be pretty heavy rain first thing, turning murky and muggy by the late morning. In the afternoon, there is the risk of heavy downpours.
"If they get unlucky, there could be an absolute cloudburst. By 2pm there could well be thundery conditions."
Forecasters at the Weather Channel said there was a risk of two weeks' worth of rain in 24 hours today.
Leon Brown, chief meteorologist, said: "After a brief drier spell in the south another developing wave is expected to move from Biscay to southern and central Britain during Thursday afternoon and evening.
"The risk of some local heavy downpours and thunderstorms in the south embedded in this. The best of the drier weather for Ascot will be mid morning with a threat of heavy showers in the afternoon."
Visitors to Ascot were sent copies of a style guide ahead of the five-day meet, after the new dress code was announced in January.
In the royal enclosure, fascinators are no longer deemed acceptable, with hats compulsory. A hat, fascinator or headpiece must be worn in the less formal grandstand area.
All women are expected to wear skirts or dresses of "modest length", according to the new guidelines, with their midriff covered.
Men now have to wear a suit and tie in the grandstand and black or grey morning dress in the royal enclosure.
About 300,000 people are expected to attend Royal Ascot – now in its 301st year – consuming some 170,000 bottles of champagne, 10,000 lobsters, 5,000 oysters and 18,000 punnets of strawberries.
