There will be plenty of dry weather, with the sunniest and hottest conditions expected in London and South East where temperatures could reach 33C (91.4F) or 34C (93.2F).
Some showery rain is possible – mainly in the North and West – but Wednesday and Thursday have the potential to break records.
Porthmadog in Wales is currently the record holder for the year – where the mercury reached 33C on 28 June.
“Wednesday will be warm but Thursday is more likely to reach 33C or 34C,” Sky News weather presenter Isobel Lang said.
“It could be the hottest day we’ve had so far, most likely in the South East.”
High levels of humidity can be expected in some places and it could also feel muggy.
It is too early to predict how long the UK heatwave will last for, but the dry, warm weather is here to stay for at least another week or so.
“There are no signs of change with the dry weather in the foreseeable future,” Ms Lang added.
Met Office forecaster Alex Burkill said: “It does look likely it will be at least 33C at some point this week, most likely on Wednesday or Thursday, in the South East and East, London, Essex, Kent, east Anglia and into Lincolnshire.
“And that would make it the hottest day we have had so far.”
Temperatures are likely to be lower elsewhere.
A West/East split in the weather is likely towards the end of the week, with the West seeing cloudier, windier conditions with some rain at times, according to the Met Office.
Central and eastern areas will remain largely fine and settle with the risk of some thundery showers, while the South and South East will be very warm or hot with warm and humid nights.
Occasional rain in the West may spread to other parts and there will be an ongoing risk of thunderstorms – particularly across central and southern and the South East.
The all-time July record was 36.7C (98.6F) set at Heathrow airport on 1 July 2015.
The hot weather could lead to thunderstorms – and it “could all happen on the same day”, Mr Burkill said.
“The heat and humidity means there is a lot of energy in the air and a high risk of intense thunderstorms. Whether or not it will happen is where the uncertainty is,” he added.
The warmer weather has led to plans for a hosepipe ban which is set to affect millions in the North West in the coming weeks.
Despite downpours in the region earlier in the week, water restrictions by United Utilities are still expected to come into force.
UK temperatures have been around 10C higher than average for this time of year.
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