The unusually warm weather has provided a boon for
British tourism destinations
The UK heatwave is expected to hit its highest temperatures on Monday, before giving way to thunderstorms and potential flooding, forecasters say.
Temperatures reached 29C (86F) in the South West on Sunday and were forecast to rise as high as 34C (93F) on Monday in some parts of the country.
But from Monday evening through to Wednesday increased humidity is set to cause storms and potential flooding.
Parts of the country will feel "a little tropical", forecasters said.
The Met Office has issued a rain warning for England, Wales and southern Scotland. The so-called yellow alert is issued to put people on notice that there may be disruptive storms.
BBC weather forecaster Laura Gilchrist said that after a tropical-feeling Monday, thunder storms would "help sweep away the hot, humid conditions".
But she warned heavy rain falling in short periods on ground hardened by the heatwave could lead to localised flooding.
The UK is in its first prolonged heatwave since 2006 - though Saturday was the first day in seven when temperatures did not exceed 30C anywhere in the country.
While the warm weather was widely welcomed initially following an indifferent start to the summer, it has had negative impacts, including grass fires in Scotland and Epping Forest in east London.
And research by the London School of Hygiene and Tropical Medicine has estimated there might have been between 540 and 760 extra deaths in England, and 60 to 100 in Wales, due to the hot weather.
The current heatwave has not matched the summer of 1976, when temperatures above 32C (89.6) were recorded on 15 consecutive days.
Hampton Waterworks in south-west London on 17 July saw the hottest day of the year so far, but our forecaster said that could well be beaten on Monday when temperatures could get as high as 34C.
The Met Office has a heatwave alert system for England which is triggered when threshold temperatures have been reached for one day and the following night, and the level depends on the likelihood of those temperatures being reached again the next day.
They are currently at level two - "alert and readiness" - for much of England, and level one - "summer preparedness and long-term planning" - for north-east England.
But it is likely that level three - "heatwave action" - will be reached again in some regions early next week, it says.
In the four years since the system was introduced, the Met Office has never used its top level-four category - "national emergency".
A Met Office spokeswoman said this would only be used after a long period of extreme hot weather leading to issues such as water shortages, infrastructure damage and deaths among "fit and healthy" people.