An amber heat-health alert has been issued for most of England, warning of "significant impacts" on the NHS.
With temperatures expected to climb to 32C (89.6F) on Wednesday and Thursday, the UK Health Security Agency (UKHSA) had initially issued a yellow heat-health alert for all of England, apart from the North East.
However, this has now been increased to an amber alert until 9pm on Sunday.
The North East now also faces a yellow heat-health alert.
The UKHSA says increased mortality across the population is likely, especially among those aged 65 or above and those with health conditions.
It also said temperatures in care settings may exceed the recommended threshold for clinical risk assessment.
An amber alert represents a situation where impacts are likely to be felt across the whole health service and potentially other sectors, with the whole population potentially at risk, according to the Met Office.
This compares to a yellow alert, which covers just those who are particularly vulnerable.
In parts of the country, temperatures are expected to be 4C higher than the Spanish holiday island Ibiza and warmer than Ayia Napa in Cyprus - 30C (86F) - as well as Athens in Greece, where it may be 27C (80.6F) on Thursday, according to the Met Office.
It could potentially beat the UK's highest temperature of the year so far of 32.2C, recorded on 10 and 25 June, despite autumn technically beginning last Friday.
This Friday could hit 31C (87.8F) in the UK but the warm conditions may change over the weekend with "no indication at the moment of another strong heatwave after this", the weather agency said.
Heat-health alerts are issued by the UK Health Security Agency during periods of hot weather when some Britons could struggle to cope.
Met Office spokesperson Oli Claydon said there will be "good sunny conditions through the week with cloudless skies" - and by Wednesday and Thursday "we could see 31, maybe 32 degrees".
He said the warmer weather is "widely spread across the UK", with the highest "probably somewhere in southeastern England spreading out to central parts of England".
It is expected to be a "tropical" night overnight on Wednesday when temperatures may not drop below 20C.
This will be most likely in the southern part of the UK and more in urban areas, he said.
"Then as we move through to Thursday, another hot day with highs of 31 to 32. And again, another warm night with potential for a tropical night on Thursday," Mr Claydon added.
Also, Saharan dust is likely to extend across parts of the UK this week, and when combined with mostly clear skies, could bring picturesque sunsets.
The summery conditions can be traced to a jet stream over the Atlantic, which until recently has been bringing mostly unsettled spells of weather to the UK.
It is continuing to move north, allowing higher pressure to build widely across the country.
There is also the influence of former tropical cyclone Franklin in the Atlantic which is amplifying the build-up of high pressure.