Festival-goers arriving at Glastonbury will face temperatures of 30C as a heatwave grips the country for a fifth day.
Music fans arriving at Worthy Farm, Somerset, on Wednesday will face a hot walk to pitch their tents.
But extensive thunderstorms are expected to sweep in from late afternoon into the night, bringing torrential downpours, frequent lightning and a chance of hail.
The UK is expected to bask in the hottest June day for 40 years with highs of 34C (93F) forecast in west London.
It could make it the hottest June day since the 35.6C (96F) recorded in Southampton on June 28 1976.
If the mercury surpasses the 31.7C (89F) recorded at Camden Square, north London, in 1936, it will be the hottest summer solstice on record.
It would also make it the fifth consecutive day that temperatures have topped 30C (86F) in the UK, the Met Office said.
The last time the country saw a similar run of hot June weather was more than 20 years ago, from June 26 to June 30 1995.
A level three amber warning, one behind a national emergency, has been in place regarding England's sweltering temperatures.
But a yellow weather warning for heavy rain spanning from 3am on Wednesday until 6am on Thursday has also been issued for south-west Scotland, England and Wales.
Bouts of rain, heavy and thundery at times, are expected to move in from the west to initially affect parts of northern England, with potentially 20mm or 30mm of rain falling in one or two hours.
More extensive thunderstorms may then spread South over a larger part of England and Wales from late in the afternoon.
Some houses, businesses and transport routes may be affected by localised flooding, while power supplies in some areas could be temporarily disrupted due to a combination of heavy rain and frequent lightning strikes.
While much of England is covered by the warning, the Met Office said it was likely that many parts would see dry and sunny weather through much of the daytime on Wednesday.
The NHS has urged the elderly, children and those with medical conditions to keep cool and hydrated during the heatwave.
Those at risk are warned to keep in the shade during the hottest part of the day between 11am and 3pm.
