Temperatures in some places could reach a scorching 35C on Friday – but the intense heat could quickly be replaced by a blast of violent thunderstorms.
Some forecasters suggest the mercury may even touch 37C (99F) – but most predictions indicate highs in the mid-30s C are likely.
Even so, Friday is expected is expected to be the hottest day of the year so far with 35C in the south and 25C in Scotland with very warm air being drawn up from the continent.
However, afternoon cloud and rain is forecast to push across Northern Ireland with a risk of thunderstorms moving northeastwards across southern counties of England.
The Met Office chief meteorologist, Andy Page, said: “The bulk of England and Wales will have dry, very warm and sunny weather to end the week with Friday likely to be the hottest day of the year so far for the UK.
“A few thunderstorms are possible later on Friday and then a cold front moves eastwards by Saturday, bringing a downturn in temperatures once again.”
Sunday should bring a combination of sunny spells and showers. In the south it remains quite warm but temperatures generally will be close to the seasonal average.
Much of next week is looking changeable and quite cool.
There are growing signs of a lengthy period of very warm and settled weather developing in the second half of August.
Met Office five-day forecast
England and Wales turning increasingly sunny and warm, after a cloudy start in places, even some rain in the north. Northern Britain cooler with some rain, although the far north brighter than recently, and a late improvement across Northern Ireland.
Rain will gradually clear from north and west Scotland to leave many parts dry with largely clear skies. A warmer night, with a gentle southerly breeze.
Hot for much of England, though coasts cooler; possible thundery showers later. Scotland and Wales very warm. Cloud and showery rain in Northern Ireland reaching some other western parts later.
Outlook for Saturday to Monday:
Cooler over the weekend with sunshine and some showers. Cloud and rain moving from the west later Sunday, across the south Monday, but drier and brighter in the north.