Parents are warned to keep children indoors up to 33C the heat.
London and south-east England can expect temperatures of up to 29C and clear skies on Sunday after parts of the country were hotter than Los Angeles and Santorini on Friday.
And the heat wave is set to continue next week when temperatures on Tuesday soar above 33C, potentially making it the warmest day of the year.
With many families expected to head to the beach and parks to enjoy the warm weather, Sheffield Children’s NHS Trust has advised parents to keep children out of the sun and ensure they stay hydrated by drinking water.” little by little and often”.
The trust wrote on Twitter: “Don’t let your children stay out in the sun for long – and never leave them in the car on a hot day. It is important to rest in the shade and find cool places.
“The first sign that a child is overheated is when they become fussy or complain of a headache. If this happens, move them to a shaded area that will be as cool as possible. Remove any clothing, give them a drink of water, and let them rest.
“Following these simple precautions means you and your children can enjoy the sun safely.”
The British Red Cross has also issued advice on how to recognize the signs of heat exhaustion and heat stroke.
The organization reminded people to “keep cool, stay hydrated and call 999 if their condition worsens”.
The UK Health Service Agency (UKHSA) and the Met Bureau have issued a level three alert for London, the east of England and the south east from 9am on Monday until 9am on Friday.
A level two warning will also be in place for the South West, East Midlands, West Midlands, North West and the Yorkshire and Humber regions.
Dr Agostino Sousa, UKHSA’s head of emergencies and public health, urged people to keep an eye on vulnerable family, friends and neighbors during the heatwave.
Temperatures soared across the country on Friday, with the mercury hitting 28.5 degrees Celsius at St James’s Park in London.
This year’s temperature was recorded at Heathrow on June 17 and is 32.7 degrees.