Amber heat health alerts are in place for large parts of England with temperatures set to rise to 30C in the South East on Saturday.
The UK Health Security Agency (UKHSA) issued the amber heat-health alert from 12pm on Friday to 6pm on Tuesday.
The warning affects East Midlands, South East, South West, East of England and London, the UKHSA said.
A yellow health warning is also in place in Yorkshire and the Humber and the West Midlands.
The alert is the second highest, behind red, on the UKHSA’s system, and the agency has warned of the possible impacts on health and social services.
The UKHSA’s weather health alert system provides early warning to the health and social care sector when adverse temperatures are likely to impact the health and well-being of the population.
On Friday, temperatures reached 29C in Coningsby, Lincolnshire, and the weather is forecast to climb over the next couple of days, according to the Met Office.
North-west England, parts of Northern Ireland and southern Scotland could see some rain on Saturday, and Scotland will see average temperatures for the time of year, but across England it could hit between 29C and 30C in the south east, the forecaster said.
On Sunday, temperatures will pick up, north-western Scotland may still see rain, but everywhere else will be warmer, with expected highs of 30-31C in the south east.
An official heatwave is recorded when areas reach a certain temperature for three consecutive days, with thresholds varying from 25C to 28C in different parts of the UK.
The East Midlands, South East, South West, East of England and London have been given a risk score of 12 out of 16, meaning there will be “significant impacts across health and social care services” due to the high temperatures.
The UKHSA warned high temperatures could cause a rise in deaths, particularly among those over 65 or with health conditions.
Younger age groups may also be affected, it said.
It also warned that there will likely be an increase in demand for health services, and indoor environments overheating, increasing the risk to vulnerable people.
There may also be issues managing medicines and an increased demand for power exceeding capacity, the UKHSA said.
Yorkshire and the Humber and the West Midlands, which have been issued a yellow warning, were given a risk score of 10 out of 16, meaning “significant impacts are possible” due to high temperatures.
Many charities have also issued warnings as the hotter weather continues this week.
The British Heart Foundation (BHF) is encouraging people with heart conditions to take precautions to minimise potential health risks during a heatwave.
Meanwhile, Age UK has urged people to check in on older relatives, friends, and neighbours to see if they need anything and to make sure they are not feeling overwhelmed by the excessive heat.
