The extreme heat advice lasts until the end of Thursday.
The Met Office’s Amber warning says people are likely to experience some adverse health effects including sunburn or heat exhaustion (dehydration, nausea, fatigue) and other heat-related illnesses.
They add there is an increased chance that some heat-sensitive systems and equipment may fail, leading to power cuts and the loss of other services to some homes and businesses.
A milder Yellow warning is also in place after a line of thunderstorms formed over southeast England on Monday night which could cause flooding, travel delays and damage to buildings.
It comes as five people have died in water-related tragedies as Britons looked for ways to escape the heat on the hottest day of the year.
Emergency services saw a wave of incidents with people getting into trouble in water on Sunday when three teenagers and a man in his 50s drowned. A man also died falling from rocks in Dorset with Police saying another body was recovered from a lake in Sheffield.
Temperatures across the UK soared on Sunday, with London reaching a mercury busting 31 degrees.
The UK is expected to get even hotter with temperatures predicted to reach 32 degrees.
Records were set in both England and Wales on Sunday, and the mercury is forecast to climb even higher on Monday.
In England on Sunday, 31.6C was recorded in Heathrow, overtaking Saturday’s record-breaking 30.3C recorded in Coton in the Elms, Derbyshire.
Wales recorded 30.2C ( in Cardiff, compared with 29.6C (85.28F) reached in Usk, Monmouthshire, on Saturday.
Watch: How to Keep Your Pets Cool During a Heat Wave