Sunday's course is expected to see competitors pass many of the city's landmarks in dry and fine conditions.
Some 50,000 participants in the mass race will cross the start line at 10am, with the temperature predicted to be about 8C.
But as the morning progresses, the sun is expected to get stronger and by the time they reach Tower Bridge, it will be around 11C.
The Met Office said it would become warmer by midday and the sun would be fairly strong.
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Met Office meteorologist Aiden McGivern said: "It will be a chilly start to the day, it soon warms up, and it will feel particularly warm in that sunshine.
"By the time we get to the Houses of Parliament, we will have temperatures of 14C or 15C.
"It stays dry and bright into the afternoon but it does look like there will be a bit more cloud cover by mid-afternoon.
"However, none of that cloud will produce any rain, the winds will be generally fairly light and from the north west. So all in all, it is looking like the weather will run smoothly."
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