The first official heatwave health warning of the year has been issued by the Met Office - for north east England and the Yorkshire and Humber region.
Health officials recommend regular application of sun protection cream
Temperatures of up to 30C (86F) and warm nights in those regions would put "extra stress" on the body, it said.
It is the first time this year temperatures have met the criteria for a Met Office Heat-Health Watch warning.
A spokesman said there was an increased risk of strokes and problems for those with heart and respiratory conditions.
High night-time temperatures of around 17C could also mean there was little time for the body to cool down and have some relief from the heat of the day.
The Met Office is forecasting southerly winds to bring some very warm air across eastern Britain over the weekend and into early next week, with increasing humidity later in the period.
A spokesman said: "This is the first warning of disruption due to heatwave.
"With many sporting and charity events taking place around the country, the strength of the sun and the combination of heat and humidity will be important factors to consider."
A warning has not been issued so far for other parts of the country, though even hotter conditions are expected elsewhere.
This is because there is a lower temperature threshold before a warning is issued in Yorkshire and the North East, as people are less used to hot weather.