Edinburgh, the Lothians, Fife, Perthshire and parts of central Scotland have now been included in the danger zone, with a hot spell building and continuing today and tomorrow.
The mercury could soar over 30C in these areas, with the Met Office warning of the impact on vulnerable people and infrastructure.
Although these areas will experience the most extreme conditions, much of the country will be baking in high temperatures – especially up the east coast to the Moray Firth.
In England, where a red warning is in place for temperatures of over 40C, there are fresh warnings of deaths in the ‘ferocious heat’ while train companies are advising people not to travel.
The public in Scotland have been warned to watch out for sunburn or heat exhaustion. Disruption to travel and services is also expected, with a possible risk of power cuts due to failures in heat-sensitive systems and equipment. There is also an increased threat of wildfires due to the intense heat and parched conditions.
The Scottish Government’s resilience minister Keith Brown said: “We are aware of the extension to the weather warnings currently in place and are receiving regular updates from partners including the Met Office and emergency services.
“Our resilience arrangements have been activated and stand ready at all times to co-ordinate a response to severe weather issues where required. We will continue to closely monitor developments.
“When temperatures increase, it’s important to monitor forecasts and follow public health advice, including staying hydrated and drinking plenty of fluids and avoiding excess alcohol.
“I would also urge people to look out for vulnerable family, friends and neighbours, as older people, those with underlying conditions and those living alone may struggle to keep cool and hydrated.”
Those planning on travelling to the red warning areas in England should expect “significant disruption”, he said.
Forecasters are predicting that Scotland could see its hottest day ever recorded, breaking the previous official high of 32.9C – set at the village of Greycrook in the Scottish Borders in August 2003.
First Minister Nicola Sturgeon tweeted: “The next couple of days are set to be exceptionally hot.
“Please take care and follow sensible advice – keep out of direct sun; stay well hydrated; and look out for others who may be suffering from the heat.”
The alert, which was expanded by the Met Office on Sunday morning now affects the Scottish Borders, Dumfries and Galloway, East Lothian, West Lothian, Midlothian, Edinburgh, Fife, Dundee and parts of Perthshire, Falkirk, Ayrshire, Stirling, Clackmannanshire and Lanarkshire.
Weather forecaster Sean Batty predicted over the weekend that a new top temperature is “likely” in the early part of the week.
“It’s likely Scotland will set a new heat record on Tuesday.
“I’ve got a feeling somewhere around Jedburgh, Melrose and Kelso could be 33-34C.”
Yesterday, College of Paramedics chief executive Tracy Nicholls said the forecast for England could result in people dying, while Deputy Prime Minister Dominic Raab suggested people should be resilient enough to “enjoy the sunshine”.
Ms Nicholls said: “This isn’t like a lovely hot day where we can put a bit of sunscreen on, go out and enjoy a swim and a meal outside.
“This is serious heat that could actually, ultimately, end in people’s deaths because it is so ferocious. We’re just not set up for that sort of heat in this country.”
Her comments were made after Mr Raab said people should take precautions ahead of the record-breaking temperatures but added that they should be able to enjoy themselves.
“Obviously there is some common-sense practical advice we are talking about – stay hydrated, stay out of the sun at the hottest times, wear sun cream – those sorts of things,” he said.
“We ought to enjoy the sunshine and actually we ought to be resilient enough through some of the pressures it will place.”
Ministers held a virtual emergency Cobra meeting on Saturday after meteorologists warned the record high temperatures could put lives at risk.
Additional contingency support for ambulance services, such as more call handlers and extra working hours, have been put in place on Monday and Tuesday.
Transport services are expected to be disrupted on both days south of the border, with Cabinet Office Minister Kit Malthouse urging people not to travel.
Avanti West Coast, Transport for London and London North Eastern Railway are also urging passengers not to travel unless it is absolutely necessary.
Meanwhile, police implored people to avoid cooling off in reservoirs, rivers or ponds after a teenage boy died while swimming with friends in a canal.
The 16-year-old got into difficulties and was last seen struggling in the water at Salford Quays in Greater Manchester at around 6.15pm on Saturday.
Dr Margaret Harris, of the World Health Organisation, has advised people to plan ahead.
“We need to take this one very seriously,” she said.
“Heatwaves are one of the most serious threats to human health, but it is often underappreciated how much of a natural disaster they can be.
“People at either end of the age span, either young or very old, cannot regulate their body temperature like those in the middle, so they need to be looked after.”
Gritters have been spreading sand on roads to avoid damage from heavy vehicles as tar begins to melt.
Meanwhile, Hammersmith Bridge in London has been wrapped in silver foil to reflect the sun’s rays and stop it overheating after cracks in the iron chains which support the 145-year-old structure opened up during a previous heatwave.
Extreme heat events do occur within natural climate variation due to changes in global weather patterns. However, experts agree that climate change, caused by human activities, has been driving an increase in the frequency, duration and intensity of these events over recent decades.
The Scottish Environment Protection Agency has again warned of an increased risk of water shortages.
In the east of Scotland, areas such as the Dee, Firth of Forth, Almond and Tyne catchment areas have been raised to moderate scarcity, which means businesses that extract water from the areas should do so only “if absolutely necessary”.