The government has issued its first warning under a new system designed to alert people to the dangers of heatwaves.
The UK Health Security Agency (UKHSA) and the Met Office have issued the first heat-health alert in six regions: London, East Midlands, West Midlands, East of England, South East and South West.
It will be in place from 9am on Friday until Monday morning.
Dr Agostinho Sousa of the UKHSA said: "In the coming days we are likely to experience our first sustained period of hot weather of the year so far, so it’s important that everyone ensures they keep hydrated and cool while enjoying the sun."
The aim of the new system is to make sure the general public is aware of the impacts of very high or very cold temperatures on people's health.
Last year the UK recorded a temperature above 40C for the first time ever in Lincolnshire, and there are predictions we could reach similar levels this summer.
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What is the new system?
The purpose of the new Heat-Health Alerting (HHA) service is to ensure people are aware of the dangers to health caused by heat waves or freezing temperatures.
Each alert with give a headline weather prediction, outline the impacts it could have, break the prediction down by region and provide links to other helpful guidance.
The new system will align with the current Met Office's National Severe Weather Warning Service by colour-coding the potential risk.
The Met Office already produces warnings for all forms of dangerous weather that include all of the potential impacts, including on travel and services.
The new HHA is specifically targeted at warning the public about the health dangers of high or low temperatures, particularly the impact it can have on the elderly.
The new system is for both professionals and the public with one of its primary goals aimed at giving the NHS enough time to prepare for any predicted extreme weather.
What do the colours mean?
The HHA has four different tiers of warnings, green, yellow, amber and red.
As expected red is the most serious with green meaning there is no alert and weather conditions are expected to have a minimum impact on health.
Yellow means some response should be triggered and will be issued when periods of heat or cold could have some impact on a vulnerable person's health, even if it is unlikely.
An amber warning means local authorities and the health service should set up a response for weather that is likely to have health impacts across the whole population.
A red warning means there is a significant risk to life for even the healthiest in the population, with all sectors expected to organise a coordinated response.
Will Lang, Head of Situational Awareness at the Met Office, said: "The updated health alerts will be complementary to, and run alongside our National Severe Weather Warnings, and will play a pivotal role in helping save lives, protect property and the economy as we all work to tackle adverse weather and climate change going forward."
How can you check the Heat-Health Alerting system?
The government has set up a dashboard where people can view if there are any heat-health alerts across the UK.