Weather forecasters have predicted heavy rain and thunderstorms after the heatwave.
The Environment Agency and Met Office have warned that 40mm of rain could fall in a few hours in London, the South East and eastern England, which could cause localised flooding.
Forecasters have also warned the public to expect hail storms.
Yesterday (Monday) was the hottest day of the year so far, which saw temperatures soar to 32.8C in Gravesend, Kent.
An Environment Agency spokesman said: "Environment Agency staff are on 24-hour alert and teams are monitoring river levels as the band of rain moves across the country. The public is encouraged to tune in to local media for weather forecasts for their area and to keep an eye out for signs of surface water flooding."
The Met Office said that water flooding could take place in some places. However, as most rivers are at low levels due to a dry spring, they are unlikely to flood.
Tom Tobler, a forecaster at MeteoGroup, said: "There will be heavy showers and thunder storms developing in South East England and East Anglia during the morning. There will be a risk of localise