The old, young and chronically ill have been warned to take care as a heatwave affecting parts of England and Wales is expected to spread across the whole UK.
The government is already urging concerned people in the West Midlands, the south-east of England and Wales to call a doctor, chemist or NHS Direct.
The north-west of England and Scotland have seen heavy rain and thunderstorms.
But the high temperatures and humidity are forecast to spread north, affecting the whole of the UK by Wednesday.
BBC meteorologist Susan Powell said Scotland and Northern Ireland were currently "pleasantly warm" but would be experiencing heatwave conditions, along with the rest of the UK, by the middle of the week.
In Lancashire, torrential rain turned some roads to rivers. In Wigan, one house was known to have been struck by lightning and the town's fire service took 70 emergency 999 calls in just 30 minutes.
Manchester was hit by torrential downpours and flash flooding which closed two lanes of the M60 motorway and damaged homes.
A Greater Manchester Fire and Rescue service spokesman said it had received more than 630 calls in four hours.
A textile factory and a house were hit by lightning, causing damage to roofs and chimneys, although no-one was injured.
In the Irlam area, firefighters rescued a 90-year-old man and six other people from two cars after they became trapped in water 4ft deep.
At Inverbervie, Aberdeenshire, 27mm of rainfall was reported.
Meanwhile, in southern and central England, the Department of Health's heatwave warning remained in place amid temperatures rising above 30C (86F).
Heat exhaustion and dehydration are the main threats to health when temperatures are high, and those at risk are being warned to keep out of the sun.
Carers, as well as staff at nursing and residential homes, are being urged to take note of the government advice.
"High temperatures can be dangerous, especially for the very young or very old or those with chronic disease," the department's website warned.
The north-west of England has seen heavy rain and flash flooding
GP Dr Sarah Jarvis said it was essential to keep fluid intakes up to counter dehydration.
"Salt intake, which is what people think about when they're sweating, is actually not the thing that's important," she said.
"It's plain old water, you don't need energy drinks. What you need is water."
The hot weather is forecast to last until Wednesday when there are set to be thundery showers, the Met Office says.
Thursday and Friday will be cooler, although temperatures are still likely to be in the mid 20s.
In north-west Europe three years ago, a heatwave caused 27,000 extra deaths.
At the same time in London, there was a 60% rise in the number of those aged over 75 who died.
People worried about their health during the heatwave are advised to talk to their GP, pharmacist, call NHS Direct on 0845 4647 or visit the website www.nhsdirect.nhs.uk.
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