London Mayor Boris Johnson has been wheeled out throughout the year in promotion of the RideLondon cycling festival.
The regular rider will take part in the 100-mile challenge on Sunday.
"My team has told me I will never make it," Mr Johnson said. "They think I am too clapped out to cycle 100 miles but the more they discourage me, the more determined I am to have a crack."
More top news
An ejector seat manufacturer is to be prosecuted over the death of Red Arrows pilot Sean Cunningham at RAF Scampton in November 2011.
Police said the "active shooter" had since been shot by officers and they were not seeking any other suspects.
Rain will continue spreading in from the southwest, remaining heavy, especially in western areas.