The UK could see six consecutive days of 30C heat in September for the first time ever, forecasters say.
Thursday was provisionally the hottest day of the year so far, with 32.6C recorded in Wisley, Surrey.
The previous highest temperature for the year was 32.2C, set during the hottest June on record.
The current heatwave is expected to last until Sunday, according to the Met Office.
Its meteorologist Simon Partridge said: “If we do see 30C all the way through until Sunday, which it looks fairly sure it will be, that will be six days in a row that we have reached 30C.
“Previously in September we’ve only reached 30C three days in a row.
Watch: UK"s hottest day of the year likely to be Saturday, Met Office says
“So although it’s not the the hottest spell of weather we’ve had in September, in terms of prolonged hot weather it is twice as long as we have previously had.”
September’s highest ever daily temperature was the 35.6C recorded on 2 September 1906 in South Yorkshire.
Partridge said there was “no chance” that the September daily temperature record would be broken in the coming days.
However, he said: “There is potential that we might get a little bit warmer over the weekend, not by a massive amount, but enough to make it the warmest day of the year so far again.
“It is always going to be around 32C, close to 33C at the maximum temperature.”
The UK Health Security Agency has issued an amber warning until 9pm on Sunday in nearly every area of England.
This means weather impacts are likely to be felt across the health service, with those aged above 65 or those with pre-existing respiratory or cardiovascular disease at greater risk.
A yellow heat alert is in place until the same time in the north east of England.
The Met Office said there could be heavy thundery showers on Sunday across England and Wales but temperatures will remain high.
The heatwave is being driven by tropical storms pushing a high pressure system over the UK, with the jet stream having moved to the north and bending into what is known as an omega blocking pattern.
Named after the Greek letter omega because of its shape, this system occurs when an area of high pressure gets stuck between two areas of low-pressure to the west and east and also slightly south.
This has brought torrential rain and flooding for Spain and Greece but hot, dry and clear conditions for the UK and central Europe.
Other parts of Europe are also experiencing a heatwave, with very high temperatures spreading across western, central and northern parts of the continent.
Temperatures reached 39C in some parts of France on Monday, while the average temperature for the country was 25.1C on the same day – making it the hottest September day ever recorded in the country.
While the current UK weather is down to high pressure in continental Europe pulling warmer air over the UK, the high temperatures in Europe have been blamed on a ‘heat dome’.
Summer in the UK has been a mixed affair this year, with a rainy July coming after the Met Office announced June was the hottest on record, with many parts of the UK officially declared in a heatwave on 13 June.
Sunday 25 June was the joint hottest day of the year so far, with temperatures reaching 32.2C in Coningsby, Lincolnshire, matching the previous high set on 10 June in Chertsey, Surrey.
Coningsby is also where the UK’s hottest ever temperature of 40.3C was recorded on 19 July last year.
What's the threshold for a heatwave in the UK?
According to the Met Office, a heatwave is "an extended period of hot weather relative to the expected conditions of the area at that time of year, which may be accompanied by high humidity".
The Met Office said the UK heatwave threshold is when a location records a "period of at least three consecutive days" with daily maximum temperatures meeting or exceeding the heatwave temperature threshold.
This threshold varies by UK county, from 25C in Northern Ireland, Scotland, Wales and the North and South West of England, to 27C and 28C in the South East of England.
The Met Office said heatwaves are more likely because of climate change.
Worldwide weather is set to be hotter than ever later this year and next year as the predicted El Niño weather event was previously confirmed as having arrived.
What about El Niño?
Experts had predicted that the arrival of the El Niño-Southern Oscillation (ENSO) would not only see general global warming but would also have a knock-on effect on global weather, with some warning of "unimaginable heat" this summer.
The natural phenomenon – a recurring climate pattern involving changes in temperature in the central and eastern tropical Pacific Ocean – was confirmed by US scientists as having started in the Pacific Ocean earlier this year, with experts predicting its cycle will make 2024 the world's hottest year.
During the last El Niño climate pattern, in 2016, the world saw its hottest year on record.
Launch of heatwave alert system
The arrival of El Niño came amid initial predictions of longer and "more intense" hot weather this summer, along with a new heatwave alert system launched in England in preparation for the spike in temperatures.
The new system was created by the Met Office and UK Health Security Agency (UKHSA) with a focus on the impact heatwaves could have on people's health.
A dedicated online platform was launched, covering an alert status for every area of England. Any "heat-health alerts" will include details on weather conditions expected over forthcoming days.
It will also give people an outline of the impacts they can expect, a brief overview of the regional impact assessment and links to additional information, advice and guidance.
The colour-coded warning system comprises green, yellow, amber and red responses – the latter of which indicates "significant risk to life for even the healthy population" and requires an emergency response.
Experts have previously warned that the heatwaves and record high temperatures seen across England last summer are likely to happen more often, last longer and be more intense in coming years and decades.
What will the long-term UK weather be like in 2023?
Early forecasts suggest that this year's El Niño could see global warming reach the crucial barrier of a 1.5C rise since pre-industrial times.
If this happens, it could lead to more heatwaves, longer hot seasons and shorter cold seasons, according to the Intergovernmental Panel on Climate Change.
Late last year the Met Office predicted temperatures in the UK during 2023 would be between 1.08C and 1.32C above the pre-industrial average – which is pretty close to this threshold.
It suggested that 2023 will be the 10th successive year that temperatures have reached at least 1C above pre-industrial levels.
Experts have suggested three consecutive years of "La Niña" events (La Niña is the opposite, cooling phase of the ESNO) have possibly "masked" the true scale of global warming in recent years.
With the UN warning that no "credible pathway" is in place to keep temperatures below 1.5C, the UK could see heatwaves above 40C more frequently.
Already this year England saw its driest February ever, followed by its wettest ever March, then record temperatures in June, signalling another year of record-breaking weather.