A heat-health alert will come into force in England later, with temperatures set to reach up to 35C (95F) in some areas this week.
It is the second alert issued in recent weeks, after extreme heat in July saw the UK exceed 40C for the first time.
The Met Office said while conditions would be below these record highs, the heatwave could be longer.
More parts of the UK are also facing hosepipe bans amid very dry conditions, as fire crews warn of wildfires.
The level three heat warning issued by the UK Health Security Agency (UKHSA) covers southern and central England from midday on Tuesday until 18:00 BST on Saturday.
The amber alert "requires social and healthcare services to target specific actions at high-risk groups", but is one stage lower than the most serous level four red warning issued in last month's heatwave.
A level two yellow alert will be in place for northern England for the same period.
Met Office meteorologist Tom Morgan said a "fairly widespread heatwave" was developing across the UK this week with the peak of the temperatures likely to be on Friday or Saturday.
Temperatures will build from 28C or 29C on Tuesday and reach the low to mid-30s from Thursday, Mr Morgan said.
"It does look like a prolonged period of dry weather and obviously that's bad news for southern England where some rain would really be useful now."
He added the West Midlands and West Country could see the highest temperatures with a maximum of around 35C, but this is uncertain.
Dr Agostinho Sousa from the UKHSA emphasised it is important to ensure that vulnerable people, like the elderly who live alone and anyone with underlying health conditions, are "prepared for coping during the hot weather".
"The most important advice is to ensure they stay hydrated, keep cool and take steps to prevent their homes from overheating," he added.
A heatwave is defined as above average temperatures being reached for three days or more. They are becoming more likely and more extreme because of human-induced climate change.
The world has already warmed by about 1.1C since the industrial era began, and temperatures will keep rising unless governments around the world make steep cuts to emissions.
We are living in the hottest period for 125,000 years, according to the UN's climate science body, the Intergovernmental Panel on Climate Change.
England had its driest July since 1935, according to the Met Office, while south-east and central southern England had the driest month since records began in 1836.
There have been calls for more hosepipe bans to be brought in, including from Environment Secretary George Eustice who urged water companies to take action on Sunday.
Southern Water already has a hosepipe ban in place for customers in Hampshire and on the Isle of Wight.
From Friday, South East Water will do the same in Kent and Sussex, while Welsh Water will bring in a ban for Pembrokeshire and Carmarthenshire later this month.
Warnings of the threat posed by wildfires have also been issued, with fire services urging people not to light bonfires or barbecues, or let off fireworks or sky lanterns.
The heatwave combined with months of dry weather have also led to warnings of growing problems for farmers, with grass not growing and irrigation water running low cited as two key concerns.
Source: BBC
Image source: MSBM
