Temperatures are predicted to hit 31C across central England on Sunday ahead of record-breaking highs next week.
The Met Office has forecast dry weather in Wales, the Midlands, the South East and south-west England on Sunday as temperatures jump to 31C in most places when an amber extreme heat warning comes in to force.
The North West and north-east England, and Scotland, meanwhile are predicted to have a cloudier day, with some showers or rain at times.
Highs of 25C are predicted in the north west of England, while the North East and most of Scotland could see highs of 23C.
Cabinet Office Minister Kit Malthouse warned transport services face “significant disruption” next week due to the heatwave, urging people not to travel.
Ministers held an emergency Cobra meeting on Saturday after meteorologists warned of record high temperatures in England next week that could put lives at risk.
After chairing the meeting, Mr Malthouse told the BBC: “Obviously the transport providers are messaging people that they should only travel if they really need to on Monday and Tuesday.
“Services are going to be significantly affected. The heat will affect rails, for example, so the trains have to run slower. There may be fewer services. People need to be on their guard for disruption.
“If they don’t have to travel, this may be a moment to work from home.”
Mr Malthouse said steps have been taken to ensure hospitals and ambulances that may come under pressure were prepared, while schools were being issued with guidance to enable them to remain open.
He added in a separate statement: “It’s important that we all continue to follow public health advice to keep cool, and take simple precautions like drinking lots of water and seeking shade, and also checking on vulnerable friends and neighbours.”
Saturday kick started the beginning of the heatwave, with Heathrow Airport and Kew Gardens, in west London, recording the highest temperature of 29.1C.
The warm weather saw increased demand for water in some parts, with South East water confirming residents in Challock and Molash, in Kent, having no water due to “continuous hot weather and significantly increased demand for water” putting “significant pressure on our network”.
A statement from the supplier said: “We’re continuing to work on restoring your supplies, and will continue to work on this overnight.
“As a precaution, we’re going to open up the bottled water station at Challock Village Hall tomorrow morning, at 8am.
“The team will be there until 7pm tomorrow night.”
Meteorologists have given an 80% chance of the mercury topping the UK’s record temperature of 38.7C, set in Cambridge in 2019, with the current heatwave set to peak on Tuesday.
Scorching temperatures are predicted for Monday, with Peterborough expected to hit 37C and Milton Keynes, Norwich and Lincoln thought to hit 36C.
Temperatures are forecast to increase by several more degrees on Tuesday – up to the mid-30s for much of England and Wales.
There is a 50% chance of temperatures reaching 40C somewhere in the UK that day, likely along the A1 corridor which runs from London to Scotland through counties including Lincolnshire, Nottinghamshire, Yorkshire and the North East.
Following the issuing of the Met Office’s first red warning for extreme heat, covering a swathe of England from London to Manchester and York on Monday and Tuesday, its chief executive Penny Endersby said people can find it hard to to know what to expect when “climate change has driven such unprecedented severe weather events”.
“Here in the UK we’re used to treating a hot spell as a chance to go and play in the sun,” she added.
“This is not that sort of weather.”
The amber warning initially covers all of England on Sunday and extends to southern Scotland and Wales from Monday until Tuesday.
London Mayor Sadiq Khan held a special meeting with key senior officials from agencies including the NHS, the Met Police, London Fire Brigade, London Ambulance Service, councils and Transport for London on Friday to ensure there is a robust plan in place to deal with the level four heat alert.
Mr Khan has urged Londoners to undertake only essential travel on Monday and Tuesday and to prepare for disruption because speed restrictions will be in place on rail and Tube networks.
A high air pollution alert for London has also been issued by the mayor for Monday.
Labour meanwhile criticised Prime Minister Boris Johnson after reports said he had failed to attend Cobra meetings.
Angela Rayner, Labour’s deputy leader, said: “Boris Johnson has gone missing in action again. He’s back to his old tricks of skipping important Cobra meetings.
“Where’s the plan for the delivery of essential services and how people will be kept safe at work, on transport, in schools, hospitals and care homes?
“The public will have no confidence in this zombie Conservative government responding swiftly and decisively to this national emergency as this disgraced Prime Minister prepares to party while Britain boils.”
The UK Health Security Agency has increased its heat health warning from level three to level four – a “national emergency”.
Level four is reached “when a heatwave is so severe and/or prolonged that its effects extend outside the health and social care system… At this level, illness and death may occur among the fit and healthy, and not just in high-risk groups,” it said.
Temperatures are expected to be especially high in London and the south of the UK (Dominic Lipinski/PA).
Britons have meanwhile been scrambling to buy fans and looser clothing to help them keep cool during the heatwave.
Retailer Toolstation said sales of fans have risen by 641% over the last week compared with the week before as tradespeople struggle to stay cool.
The company also said sales of builders’ shorts have increased by 50% and t-shirts by 35%.
Fire brigades, including South Wales Fire And Rescue Service, Scottish Fire And Rescue and London Fire Brigade, have issued safety warnings, urging people to act responsibly.
They warn people to disposing of barbecues, lit cigarettes and glass bottles responsibly, to not burn any rubbish such as garden waste and use local authority services instead, and that barbecues should not be used on balconies or near sheds, fences, trees, shrubs and garden waste to avoid anything catching alight.
They also urge people who are cooling off in waterways to be aware of cold-water shock.