An amber warning for extreme heat has been issued for parts of England and Wales, as Thames Water says it will announce a hosepipe ban "in the coming weeks".
The Met Office has issued the warning for four days from Thursday to Sunday, saying "adverse health effects" are likely to be experienced by some vulnerable people.
Thames Water says it is planning to announce a temporary hosepipe ban.
It said: "Given the long term forecast of dry weather and another forecast of very hot temperatures coming this week we are planning to announce a temporary use ban in the coming weeks."
It urged its customers to "only use what they need for their essential use."
Health alerts have been announced for England, with a level 3 in place from 12pm on Tuesday until 11pm on Saturday.
Level 3 is triggered when certain temperatures are reached and triggers specific actions targeted at high‑risk groups. Level 4 - a red warning - is the most serious designation.
The Met Office says temperatures are likely to rise into the low to mid-30s in central and southern parts of the UK on Friday and Saturday - but will not be as extreme as the record-breaking heat in July which saw the mercury climb above 40C.
Outside the hottest areas, much of England and Wales and southeast Scotland could see temperatures widely in the high 20s, with a chance of a few spots seeing temperatures into the low 30s, the Met Office said.
Watch: Climate change is making droughts worse as our atmosphere heats up, scientists say
Scotland and Northern Ireland will also see temperatures in the high 20s and could reach official heatwave criteria by Friday, the forecasters said.
Warming conditions in the UK have prompted the Met Office to raise the temperatures that have to be reached for an official heatwave for eight English counties.
In its warning, the Met Office said the wider population could experience adverse health effects including sunburn or heat exhaustion (dehydration, nausea, fatigue) and other heat related illnesses.
Additionally, it said increased visits to coastal areas, lakes, rivers and other beauty spots will lead to an increased risk of water safety and fire-related incidents.
Some delays to road, rail and air travel are possible, the Met Office added.
With the latest heatwave coming after months of low rain, which have left the countryside and urban parks and gardens tinder-dry, households in some areas are being urged not to light fires or have barbecues.
The Met Office's fire severity index, an assessment of how severe a fire could become if one were to start, is very high for most of England and Wales, and will reach "exceptional" for a swathe of England by the weekend.
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Scientists say the likelihood of droughts occurring is becoming higher due to climate change, driven by greenhouse gas emissions from burning fossil fuels and other human activities.
Dr Leslie Mabon, lecturer in environmental systems at The Open University, said: "Above all else, the drought risk we are seeing in the UK is a reminder that we urgently need to tackle the problem at source: this means reducing emissions from fossil fuels to limit the extent of harmful climate change we will face."