Organisers of the Prudential RideLondon cycling festival have offered advice for riding during the heatwave.
More than 100,000 cyclists will take to the roads of London and Surrey over the weekend for professional races, cycling challenges and a mass bike ride.
Temperatures in the capital soared to over 35C this week, but the mercury is predicted to drop on Saturday.
Riders have been advised to reduce their speed and not overexert themselves in the hot weather.
Director Hugh Brasher said diversion routes would be available so riders do not have to climb Leith and Box Hills.
"We have reminded riders to come prepared with full water bottles and there is plenty of water and electrolytes available on the route," he said.
Organisers will also increase the water supply to all drinks stations and hubs and increase the supply of ice to first aid stations.
Events for both amateurs and professionals will take place during the festival, which has been held annually since 2013.
Transport for London (TfL) has warned that numerous roads, including 13 river crossings, will be closed at different times during the event.
Will Norman, London's walking and cycling commissioner, said he hoped the festival will "help introduce young people to cycling and encourage others to get back on their bikes or cycle more".
Travel disruption during Prudential RideLondon
- Drivers are advised to avoid central London between 05:00 and 19:30 BST on Saturday while the FreeCycle takes place
- On Sunday, roads will be closed in parts of east, central and south-west London and parts of Surrey, with drivers advised to avoid the area between 04:00 and 20:00
- Buses which travel on affected routes will be on diversion or will finish early
- People are advised to use Tube and rail services over the weekend
Find full travel advice and a list of road closures for Saturday here.
Find full travel advice and a list of road closures for Sunday here.