Temperatures are set to reach 32C (90F) across many southern parts of Britain over the next few days, leading to the chance of severe thunderstorms.
Today, England and Wales will be mainly dry with spells of warm sunshine and it will feel very warm, especially across the southeast.
Scotland, Northern Ireland and the Republic will also have sunny spells, however western Scotland may see some scattered showers.
Temperatures will widely reach the mid to high 20s across England and Wales, and up to 30C (86F) is possible in the southeast.
However, Scotland and Ireland will be several degrees cooler.
By this evening, thundery showers will spread into Kerry and Cork and move northwards through the night across most of the Republic of Ireland as well as parts of southwest England, Wales, the Midlands and central and southern England.
The heaviest and most torrential downpours are likely over Ireland. It will be a warm and muggy night as well, especially in the south.
Friday will be another very warm day across England and Wales with high humidity.
The showers and thunderstorms will continue spreading northwards into Northern Ireland, northern England and Scotland.
As the showers clear, long spells of sunshine will develop in the south. Temperatures will be high again, up to 32C (90F) in the south.
However, from Friday night and into Saturday the Met Office has issued a weather warning for heavy thundery showers for England, Wales and southern Scotland
It says frequent lightning, large hail and strong gusts are possible - as well as flooding.
The hot weather has brought health warnings from authorities - especially for the elderly, the young and those with serious illnesses.
Dr Paul Cosford, director for health protection at PHE, said: "Older people and those with long-term illnesses are particularly vulnerable to the effects of very hot weather, so it's important to look out for them and keep indoor areas as cool as possible."
The Met Office has declared a level 2 heat-health alert, which is activated when there is at least a 60% chance of temperatures being high enough on at least two consecutive days and into the night to have an impact on health.
The reason for the hot conditions is southerly winds dragging up very warm and humid air, mainly from Iberia where temperatures are close to 40C (104F) in the south of Spain.
Next week's temperatures are likely to be a little lower, however it will still feel very warm and there will still be some thunderstorms.
