The bicycle race that will see thousands ride from London to Surrey has been shortened by 14 miles amid a forecast of heavy rain.
Organisers of RideLondon said they "reluctantly" made the decision to cut the race from 100 miles to 86 after information from the Met Office as the effects of Hurricane Bertha's remnants are felt in the UK.
The route will now miss out a section covering Leith Hill and Box Hill in Surrey due to predictions of heavy rain.
Event director Hugh Brasher said the safety of the cyclists, who will include 2012 Tour de France and Olympic gold medal-winner Sir Bradley Wiggins, was the most important consideration.
"This decision has been made in the light of detailed information from the Met Office and with full agreement of local authorities and other agencies," said Mr Brasher.
"The current weather forecast is for localised, heavy rain. Our primary concern is for the safety of all participants, volunteers and staff.
"Please follow the instructions of our stewards and marshals at all times.
"Please ride appropriately in these wet and windy conditions: leave more space between you and your fellow riders and moderate your speed."
Sir Bradley will race for Team Sky - which dropped him for this year's Tour - in the Prudential RideLondon-Surrey Classic this afternoon with a host of big-name pro cyclists.
More than 22,000 amateur riders are taking part in the race, which finishes on the Mall outside Buckingham Palace.
Sir Bradley, 34, said: "I always enjoy competing in front of home fans and have great memories of riding this route at the Olympics.
"The support we got then was unbelievable, and it was the same again at the Tour of Britain last year, so this promises to be a really special day."
He will be joined in the pro race by 2012 World Champion Philippe Gilbert, Belkin Pro Cycling duo Laurens ten Dam and Steven Kruijswijk, who were 19th and 15th respectively in this year's Tour de France, plus five medallists from the Commonwealth Games, gold medallists Luke Davidson and Tom Scully, Shane Archbold, who won a gold and bronze, and bronze medal-winners Scott Thwaites and Aaron Gate.
The professional riders head out from the Olympic park in east London at 1pm, but before they blitz around the city streets and rural roads at high speed, thousands of plucky amateurs will attempt the same gruelling route.
Yesterday an estimated 60,000 people took to the streets of London for the RideLondon Freecycle.
The event saw a ten-mile circuit of normally busy London roads cleared for cyclists, taking in some of the capital's top landmarks, including Buckingham Palace, the Houses of Parliament, St Paul's Cathedral and the Tower of London.