The Meteorological Office said a Level 2 heat-health alert has been issued for a large part of southern and central England, with a Level 1 alert in place for northern England.
Some parts of England are forecast to experience a heatwave this week with temperatures predicted to jump to 28°C in the north-east and north-west, and 30°C in the south-east on Friday.
But they said the heatwave was likely to be short.
On the four-level alert scale, which is designed to help healthcare workers manage through periods of extreme temperatures, Level 1 is the lowest and is the minimum state of vigilance for summer months.
Level 2, called alert and readiness, is issued as soon there is a 60 per cent risk that temperature thresholds will be reached in one or more regions on at least two consecutive days and the night between.
A heatwave is defined as three consecutive days with daily maximum temperatures meeting or exceeding the heatwave temperature threshold.
The threshold varies in each county.
“Temperatures are forecast to reach 30°C in some parts of the south on Friday, and we want everyone to enjoy the hot weather safely when it arrives and be aware of good health advice for coping with warmer conditions," said Agostinho Sousa, head of extreme events at the health agency.
“During periods of hot weather it is especially important to keep checking on those who are most vulnerable, such as older people and those with heart or lung conditions.
“Make sure to look out for signs of heat exhaustion and follow our simple health advice to beat the heat.”
Met Office deputy chief meteorologist Dan Rudman said the temperature predictions were unusual for this time of year.
“Many areas will also see some warm nights with minimum temperatures expected to be in the high teens or even low 20s for some overnight,” Mr Rudman said.
“The heat is a result of a mix of home-grown warming in the day due to high pressure, as well as a southerly airflow introducing some of the warm air from the continent to UK shores.”
The highest temperature reached in the UK so far this year was 27.5°C at Heathrow on May 17.
An Environment Agency spokesman said there was a low risk of drought but warned more hot, dry weather could put pressure on some areas.
The British Red Cross, meanwhile, has encouraged people to protect themselves and to check in with vulnerable friends, family and neighbours during the rising temperatures.
The charity has given advice to help keep people healthy, urging them to drink plenty of fluids but avoid excessive alcohol consumption, wear sun cream and keep workplaces and homes cool.
The Royal National Lifeboat Institution has also issued safety advice to members of the public.
The organisation has said that people should go only to beaches with lifeguards and swim between the red and yellow flags.
“With summer arriving and a heatwave forecast, we want to remind everyone to stay safe at the coast," said Gabbi Batchelor, water safety education manager at the RNLI.
“It is important that anyone visiting the coast understands the risks of the environment.
"It can be very unpredictable, particularly during early summer when the risk of cold water shock significantly increases, as air temperatures warm but water temperatures remain dangerously cold.
“If you get into trouble in the water, float to live. Lean back, using your arms and legs to stay afloat.
"Control your breathing, then call for help or swim to safety. In a coastal emergency, call 999 or 112 for the coast guard.”