An amber heat health alert has been issued for large parts of the UK as forecasters predict that Wednesday could be the hottest day of the year.
The Met Office is watching a number of September heat records as a mini-heatwave is set to bring temperatures higher than those in Ibiza.
On Wednesday temperatures could reach 33C, beating the current high of 32.2C, seen back in June.
The UK Health Security Agency (UKHSA) and Met Office had previously issued a yellow heat warning, but on Tuesday upgraded it to an amber alert across eight regions of England including London.
The heat-health alert is in place until 9pm on Sunday.
The other regions included in the amber alert are the South-East, South-West, North-West, East Midlands, West Midlands, east of England, and Yorkshire and the Humber.
The alert means the weather impacts “are likely to be felt across the whole health service”.
The UKHSA warned those aged 65 and above or with pre-existing health conditions such as those with heart or lung conditions, could be at increased risk.
Dr Agostinho Sousa, Head of Extreme Events and Health Protection at UKHSA, urged people to “take sensible precautions while enjoying the sun and look out for those who are more vulnerable to the effects of heat”.
“We advise you to check on older family members, friends, or neighbours and those with heart or lung conditions,” he said.
“Staying hydrated and keeping cool is crucial for everyone during hot weather, while enjoying the sun.”
The heatwave is set to bring daytime temperatures of 33C as it peaks on Wednesday and Thursday, when the UK is set to be 5C hotter than the Spanish island of Ibiza.
The Met Office has said temperatures could also be warmer than Ayia Napa in Cyprus (30C) and Athens in Greece, where it could be 27C on Thursday.
Met Office meteorologist Tom Morgan said by the end of Tuesday it would “officially be a heatwave”.
He said: "We may well be close to some record-breaking temperatures in the next few days.
"The daytime maximum temperatures are a little less likely to be broken but nonetheless it will be hot so we are watching a couple of records.
"The most likely record that we could see broken is the highest overnight temperature for Wales. Currently the September highest overnight temperature for Wales stands as 20.5C.
"There is a possibility either tonight or tomorrow night we could see a temperature not fall below that value in parts of Wales.
"The highest UK September temperature we've ever seen still stands at 35.6C.
"So we are very unlikely to see temperatures quite that high but probably a 33 is on the cards either tomorrow or maybe also on Thursday, so we are not too far away from the UK record either."
The heatwave will also bring “uncomfortably warm” nights, especially in the south.
It is expected to bring ‘tropical nights’, which occur when the temperature does not drop below 20C.
The record for the highest overnight minimum temperature for September, which currently stands at 21.7C, could be broken on Wednesday and Thursday nights in particular, said forecasters.
“(It could be) a warm night overnight on Wednesday, with the potential for temperatures not dropping below 20 degrees, which is what we term a tropical night,” said Met Office spokesperson Oli Claydon.
“That’s most likely in the southern half of the UK and more likely in urban areas where the temperatures obviously stay up that little bit higher overnight.
“Then as we move through to Thursday, another hot day.. and again, another warm night with potential for a tropical night on Thursday.”
The UK is not understood to have recorded consecutive tropical nights in September before.
The forecaster explained tropical storms in the far western Atlantic and deep areas of low pressure have helped to amplify the jet stream over the Atlantic Ocean and has led to high pressure “dominating over the UK”.
But the weather conditions could change over the weekend and Mr Claydon said there is “no indication at the moment of another strong heatwave after this”.
He added: “Through the weekend, we start to see some heavy, potentially thundery, showers developing but (they are) only isolated. There is a little bit of uncertainty as we start to get that far ahead.”
Average temperatures are expected to return by the middle of next week.
Heatwaves are becoming more likely and more extreme because of climate change.
Last year the UK recorded temperatures above 40C for the first time. Scientists said that would have been “virtually impossible without climate change”.
The Met Office has also explained the reason for some striking sunsets across the UK in recent days. Forecasters say it is due to “Saharan dust” and it is due to cover parts of the country later this week.
