The Met Office has issued a level 3 heatwave action plan lasting from Saturday until Bank Holiday Monday.
The amber warning came on Friday morning, with forecasters urging people to stay out of the sun, keep drinking fluids and take care of elderly people living on their own. It states: “There is a 90% probability of heat health criteria being met between 0900 on Saturday and 0900 on Monday in parts of England.”
It adds: “Keep your home as cool as possible – shading windows and shutting them during the day may help. Open then when it is cooler at night.”
From Friday afternoon, south-easterly winds will draw warm air from Europe towards the UK, bringing rising temperatures along with dry and settled weather.
Temperatures will rise to 26C in London and 24C in Cardiff and the rest of the UK will be largely dry with lots of sunshine, according to the Met Office.
In Scotland, however, there will be a series of showers with the north-west being the most affected area.
By Monday, temperatures may reach 33C in southern England - which would set a new record for the late bank holiday weekend.
The highest record temperature for the late August bank holiday weekend is 31.5C at Heathrow Airport in 2001.
The sunny spell could set a swathe of new records around the country, with the current hottest late summer bank holiday in Wales standing at 27.3C at Velindre, Powys, in 2013.
The Northern Ireland record is 27C recorded in Knockaraven, Co Fermanagh in 2003.
Monday is not a bank holiday in Scotland.
Friday is expected to see five million drivers embarking on journeys for a holiday or day-trip ahead of the bank holiday weekend.
The roads are expected to be busiest after 11am.